Current amplifier circuit, integrator, and ad converter

ABSTRACT

In one embodiment, a current amplifier circuit includes a first transistor, a first resistor, a second transistor, a second resistor, a first passive element, and a control circuit. The first transistor has a first terminal, a second terminal, and a control terminal. The first resistor has one end connected to the first terminal of the first transistor. The second transistor has a first terminal, a second terminal, and a control terminal. The second resistor has one end connected to the first terminal of the second transistor. The first passive element is connected between the first terminals of the first transistor and the second transistor. The control circuit controls at least one of voltage at the control terminals of the first transistor and the second transistor such that the voltage at the other end of the first resistor becomes equal to the voltage at the other end of the second resistor.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2013-235298, filed on Nov. 13, 2013, and prior Japanese Patent Application No. 2014-215707, filed on Oct. 22, 2014, the entire contents all of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a current amplifier circuit, integrator, and AD converter.

BACKGROUND

In the related art, there are cases in which a sensor that detects a state of an object by changing a resistance value has a small change rate of the resistance value, such as one percent, corresponding to a state change. In the case of using such a sensor, a change of resistance value is converted to an electrical signal and then the electrical signal is amplified by an amplifier circuit. In the case where the change of the resistance value is converted to current, a current amplifier circuit is used. However, the current amplifier circuit in the related art, there is a problem in which most of bias current is consumed.

For example, it is known that a current mirror circuit is used as the current amplifier circuit in the related art. In the case of using this kind of current amplifier circuit, the change of the resistance value is converted to the current at first. Provided that the change rate of the resistance value is Δ_(R) and the bias current is I_(b), the converted current becomes: I_(b) (1+Δ_(R)). In other words, the change of the resistance value is converted to the current in which signal current I_(b)Δ_(R) is superimposed on the bias current I_(b).

The converted current is received in the current mirror circuit and amplified by a multiple of a device size ratio. Here, when the device size ratio is K, the current amplified by the current mirror circuit is KI_(b)(1+Δ_(R)). In this case, a ratio between the bias current consumed in an entire circuit and signal current to be output (hereinafter referred to as “ratio between the bias current and the signal current”) is: KI_(b)Δ_(R)/(I_(b)+KI_(b))=KΔ_(R)/(1+K). Since normally K is a value of one or larger, the ratio between the bias current and the signal current KΔ_(R)/(1+K) becomes Δ_(R) or less. Thus, the signal current becomes K-fold, but the bias current is also increased, thereby not improving the ratio between the bias current and the signal current.

As a method of improving the above situation, there is a proposed method in which the ratio between the bias current and the signal current is improved by subtracting predetermined current I_(b1) from the converted current I_(b)(1+Δ_(R)) before being received in the current mirror circuit. According to such a current amplifier circuit, output of the current mirror circuit becomes: K (I_(b)−I_(b1)+I_(b)Δ_(R)). Therefore, the ratio between the bias current and the signal current becomes: KI_(b)Δ_(R)/{I_(b)+K (I_(b)−I_(b1))}.

For example, in the case where I_(b)=200 μA, Δ_(R)=0.01 (=1%), I_(b1)=197.8 μA, and K=80, total bias current becomes 376 μA (=200 μA+80×(200 μA−197.8 μA)), and the signal current becomes 160 μA (=80×200 μA×0.01). Therefore, the ratio between the bias current and the signal current is: 160/376=0.43.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a current amplifier circuit according to a first embodiment;

FIGS. 2 to 17 are diagrams illustrating respective working examples according to the first embodiment;

FIG. 18 is a diagram of a current amplifier circuit according to a second embodiment;

FIG. 19 is a diagram illustrating an example of a passive element in FIG. 18;

FIG. 20 is a diagram illustrating another example of the passive element in FIG. 18;

FIG. 21 is a diagram illustrating a current amplifier circuit according to a third embodiment;

FIG. 22 is a diagram illustrating a current amplifier circuit according to a fourth embodiment;

FIG. 23 is a diagram of a current amplifier circuit according to a fifth embodiment;

FIGS. 24A to 24F are explanatory diagrams illustrating signal changes caused by operation of a switching circuit;

FIG. 25 is a diagram illustrating a first working example according to the fifth embodiment;

FIG. 26 is a diagram of a current amplifier circuit according to a sixth embodiment;

FIG. 27 is a diagram illustrating a first working example according to the sixth embodiment;

FIG. 28 is a diagram of a current amplifier circuit according to a seventh embodiment;

FIG. 29 is a diagram illustrating a first working example according to the seventh embodiment;

FIG. 30 is a diagram illustrating a second working example according to the seventh embodiment;

FIG. 31 is a diagram of a current amplifier circuit according to an eighth embodiment;

FIG. 32 is a diagram illustrating an example of a passive element in FIG. 31;

FIG. 33 is a diagram illustrating an integrator according to a ninth embodiment;

FIGS. 34 to 39 are diagrams illustrating respective working examples according to the ninth embodiment;

FIG. 40 is a diagram illustrating an AD converter according to a tenth embodiment;

FIG. 41 is a diagram illustrating a first working example according to the tenth embodiment;

FIG. 42 is a diagram illustrating a second working example according to the tenth embodiment;

FIG. 43 is a diagram of a current amplifier circuit according to an eleventh embodiment;

FIG. 44 is a diagram illustrating respective working example according to the eleventh embodiment;

FIG. 45 is a diagram of a current amplifier circuit according to a twelfth embodiment;

FIG. 46 is a diagram illustrating respective working example according to the twelfth embodiment;

FIG. 47 is a diagram of a current amplifier circuit according to a thirteenth embodiment;

FIG. 48 is a diagram illustrating an example of a voltage-current conversion circuit in FIG. 47;

FIG. 49 is a diagram of a current amplifier circuit according to a fourteenth embodiment;

FIG. 50 is a diagram illustrating an example of a voltage-current conversion circuit in FIG. 49;

FIG. 51 is a diagram of a current amplifier circuit according to a fifteenth embodiment; and

FIGS. 52 and 53 are diagrams illustrating another example of the current amplifier circuit in FIG. 51.

DETAILED DESCRIPTION

Embodiments will now be explained with reference to the accompanying drawings. The present invention is not limited to the embodiments.

In one embodiment, a current amplifier circuit includes a first transistor, a first resistor, a second transistor, a second resistor, a first passive element, and a control circuit. The first transistor has a first terminal, a second terminal, and a control terminal. The first resistor has one end connected to the first terminal of the first transistor. The second transistor has a first terminal, a second terminal, and a control terminal. The second resistor has one end connected to the first terminal of the second transistor. The first passive element is connected between the first terminal of the first transistor and the first terminal of the second transistor. The control circuit controls at least one of voltage at the control terminal of the first transistor and voltage at the control terminal of the second transistor such that the voltage at the other end of the first resistor becomes equal to the voltage at the other end of the second resistor.

The current amplifier circuit amplifies a differential signal received from input terminals T₁ (first input terminal) and T₂ (second input terminal), and executes output from output terminals T₃ and T₃. In the following, a current amplifier circuit formed of a MOS transistor will be described, but the current amplifier circuit can be also formed of a bipolar transistor. The current amplifier circuit can be formed of the bipolar transistor by substituting the MOS transistor described below with the bipolar transistor and further substituting a source terminal with an emitter terminal, a drain terminal with a collector terminal, and a gate terminal with a base terminal.

Further, in the following, an embodiment mainly using an N-channel MOS transistor will be described, but it is also possible to configure a current amplifier circuit mainly using a P-channel MOS transistor or a PNP type bipolar transistor. The current amplifier circuit mainly using the P-channel MOS transistor or the PNP type bipolar transistor can be formed by substituting the N-channel (P-channel) MOS transistor described below with the P-channel (N-channel) MOS transistor or with the PNP type (NPN type) bipolar transistor, and connecting a terminal connected to a ground (power source) to the power source (ground).

(First Embodiment)

In the following, a current amplifier circuit according to a first embodiment will be described with reference to FIGS. 1 to 17. Here, FIG. 1 is a diagram illustrating the current amplifier circuit according to the first embodiment. As illustrated in FIG. 1, the current amplifier circuit according to the present embodiment includes a transistor M₁, a resistor R₁, a transistor M₂, a resistor R₂, a passive element Z₁, and a control circuit.

The transistor M₁ (first transistor) is an N-channel MOS transistor (hereinafter referred to as “NMOS transistor”). The transistor M₁ has a source terminal (first terminal) connected to the resistor R₁ and the passive element Z₁ at a node N₃, a drain terminal (second terminal) connected to an output terminal T₃, and a gate terminal (control terminal) connected to the control circuit. The voltage at the gate terminal of the transistor M₁ is controlled by the control circuit, and drain current I_(M1) flows in the transistor M₁.

One end of the resistor R₁ (first resistor) is connected to the source terminal of the transistor M₁ and the passive element Z₁ at the node N₃, and the other end is connected to the control circuit at a node N₁. The resistor R₁ may be a fixed resistor having a constant resistance value, or may also be a sensor device that detects a state of an object by change of the resistance value.

The transistor M₂ (second transistor) is an NMOS transistor. The transistor M₂ has a source terminal (first terminal) connected to the resistor R₂ and the passive element Z₁ at a node N₄, a drain terminal (second terminal) connected to an output terminal T₄, and a gate terminal (control terminal) connected to the control circuit. The voltage at the gate terminal of the transistor M₂ is controlled by the control circuit, and drain current I_(M2) flows in the transistor M₂.

One end of the resistor R₂ (second resistor) is connected to a source terminal of the transistor M₂ and the passive element Z₁ at the node N₄, and the other end is connected to the control circuit at a node N₂. The resistor R₂ may be a fixed resistor having a constant resistance value, or may also be a sensor device that detects a state of an object by change of the resistance value. Meanwhile, in the case where the resistor R₁ has the fixed resistor, preferably the resistor R₂ has the fixed resistor having a resistance value equal to the resistance value of the resistor R₁ (R₁=R₂). Additionally, in the case where the resistor R₁ is the sensor device, preferably the resistor R₂ is the sensor device having a same absolute value with an opposite sign of a change rate of the resistance value of the resistor R₁ as next: (R₁=R_(C)(1−Δ_(R)), and R₂=R_(C)(1+Δ_(R))).

The passive element Z₁ (first passive element) is connected to the source terminal of the transistor M₁ and one end of the resistor R₁ at the node N₃, and further connected to the source terminal of the transistor M₂ and one end of the resistor R₂ at the node N₄. The passive element Z₁ may be formed of resistor or capacitor. For example, the passive element Z₁ may be formed of resistor and capacitor connected in series. In this case, when a frequency of an input signal is low, impedance element Z₁ of the passive element Z₁ is substantially determined by a capacitance component, and when the frequency is high, the impedance element Z₁ is substantially determined by a resistance component. Thus, the impedance element Z₁ may include frequency characteristic.

The control circuit is connected to the gate terminal of the transistor M₁, the gate terminal of the transistor M₂, the other end of the resistor R₁, the other end of the resistor R₂, and the input terminals T₁ and T₂. The control circuit controls at least one of the voltage at the gate terminal of the transistor M₁ and the voltage at the gate terminal of the transistor M₂ such that voltage V₁ (first predetermined voltage) at the other end of the resistor R₁ becomes equal to voltage V₂ (second predetermined voltage) at the other end of the resistor R₂, more specifically, the voltage V₁ at the node N₁ becomes equal to the voltage V₂ at the node N₂. Further, the control circuit executes control such that current I₁ received from the input terminal T₁ is applied to the resistor R₁ and current I₂ received from input terminal T₂ is applied to the resistor R₂.

Next, operation of the current amplifier circuit according to the present embodiment will be described. In the following description, note that the resistor R₁ and the resistor R₂ are the fixed resistor: R₁=R₂=R_(C), and the passive element Z₁ is resistor. Further, the voltage V₁ and voltage V₂ at the nodes N₁ and N₂ are controlled to be V_(C) by the control circuit (V₁=V₂=V_(C)). Additionally, the current I₁ and the current I₂ are differential signals which are: I₁=I_(b)(1+Δ_(R)), and I₂=I_(b)(1−Δ_(R)). In other words, a description will be given for a case in which the current amplifier circuit is connected to the sensor device, and signal current is differentially received from the terminals T₁ and T₂.

When the current I₁ and current I₂ are received from the input terminals T₁ and T₂, voltage V₃ at the node N₃ and voltage V₄ at the node N₄ become: V₃=V₁+R₁I₁ and V₄=V₂+R₂I₂. Here, in the case of plugging following values (R₁=R₂=R_(C), I₁=I_(b)(1+Δ_(R)), I₂=I_(b)(1−Δ_(R)), and V₁=V₂=V_(C)) into the mentioned formula, the voltage V₃ and V₄ become: V₃=V_(C)+R_(C)I_(b)+R_(C)I_(b)Δ_(R) and V₄=V_(C)+R_(C)I_(b)−R_(C)I_(b)Δ_(R).

Accordingly, the voltage of V₃−V₄=2R_(C)I_(b)Δ_(R) is applied to the passive element Z₁, and following current I_(Z1) flows: I_(Z1)=2R_(C)I_(b)Δ_(R)/Z₁. Due to this, the current I_(M1) flowing in the transistor M₁ becomes: I_(M1)=I₁+I_(Z1)=I_(b)+Δ_(R)I_(b)+2R_(C)I_(b)Δ_(R)/Z₁. The current I_(M2) flowing in the transistor M₂ becomes: I_(M2)=I₂−I_(Z1)=I_(b)−Δ_(R)I_(b)−2R_(C)I_(b)Δ_(R)/Z₁. According to the present embodiment, the current I_(M1) and current I_(M2) are the output signals to be output from the output terminals T₃ and T₄.

Here, note that bias current I_(B) flowing in the entire circuit is a sum of bias current contained in the output signals (current I_(M1) and I_(M2)), and therefore the bias current I_(B) becomes: I_(B)=2I_(b). This is equal to a sum of the bias current contained in the input signals (current I₁ and I₂). In other words, according to the present embodiment, the bias current contained in the input signal is not amplified.

On the other hand, signal current I_(SIG) output from the current amplifier circuit is a difference between the two output signals (current I_(M3) and I_(M4)), and therefore the signal current becomes: I_(SIG)=I_(M1)−I_(M2)=2Δ_(R)I_(b)+4R_(C)I_(b)Δ_(R)/Z₁.

For example, in the case of R_(C)=4 kΩ, Δ_(R)=0.01, I_(b)=100 μA, and Z₁=100Ω, the signal current contained in the input signal becomes: I₁−I₂=2I_(b)Δ_(R)=2 μA. The signal current I_(SIG) contained in the output signal becomes: I_(SIG)=162 μA. In other words, the signal current is amplified 81-fold. Further, the bias current I_(B) consumed in the current amplifier circuit becomes: I_(B)=200 μA, and the ratio between the bias current and the signal current becomes 0.81. Thus, the ratio between the bias current and the signal current in the current amplifier circuit according to the present embodiment becomes larger than the ratio between the bias current and the signal current in the current amplifier circuit according to the related art.

As described above, in the current amplifier circuit according to the present embodiment, the voltage (V₃−V₄) corresponding to the signal current contained in the input signal is applied to the passive element Z₁, thereby allowing the current I_(Z1) corresponding to the signal current to flow in the passive element Z₁. The signal current is amplified and amplification of the bias current can be suppressed by adding the current I_(Z1) to the input signals (I₁, I₂) and generating the output signals. In this manner, the ratio between the bias current and the signal current can be improved and power consumption in the current amplifier circuit can be suppressed, compared to the current amplifier circuit in the related art.

Meanwhile, in the current amplifier circuit, only the bias current may be received from one of the input terminals T₁ and T₂. For example, the input signals may be: I₁=I_(b)(1+Δ_(R)) and I₂=I_(b). In this case, the signal current to be received is: I₁−I₂=I_(b)Δ_(R)=1 μA. The signal current to be output is: I_(SIG)=Δ_(R)I_(b))+2R_(C)I_(b)Δ_(R)/Z₁=81 μA. In other words, an amplification factor of the signal current is same as the case where both input signals contain the signal current. In this case, the bias current in the entire current amplifier circuit is: I_(B)=2I_(b)=200 μA. Therefore, the ratio between the bias current and the signal current becomes 0.405.

(First Working Example)

FIG. 2 is a diagram illustrating a first working example according to the present embodiment. In the present working example, the current amplifier circuit includes an amplifier circuit A₁ and an amplifier circuit A₂ as the control circuits. The amplifier circuit A₁ (A₂) has a positive input terminal from which predetermined voltage V_(C) is applied, and a negative input terminal connected to the other end of the resistor R₁ (R₂) at the node N₁ (N₂), and an output terminal connected to the gate terminal of the transistor M₁ (M₂). Further, the input signals I₁ and I₂ are received from current sources I₁ and I₂. Note that, in the following, the input terminals T₁ and T₂ correspond to the nodes N₁ and N₂ unless otherwise specified.

Now, operation of the control circuit according to the present working example will be described. The amplifier circuit A₁ forms a feedback circuit together with the transistor M₁ and the resistor R₁, and executes feedback control such that voltage at the positive terminal and negative terminal in the amplifier circuit A₁ become equal. More specifically, the amplifier circuit A₁ controls the voltage at the gate terminal of the transistor M₁ such that the voltage at the negative input terminal (voltage V₁ at the node N₁) becomes equal to predetermined voltage V_(C). In the same manner, the amplifier circuit A₂ forms the feedback circuit together with the transistor M₂ and the resistor R₂, and executes the feedback control such that voltage at the positive terminal and negative terminal in the amplifier circuit A₂ become equal. More specifically, the amplifier circuit A₂ controls the voltage at the gate terminal of the transistor M₂ such that the voltage at the negative input terminal (voltage V₂ at the node N₂) becomes equal to predetermined voltage V_(C). With this configuration, the voltage V₁ and V₂ at the nodes N₁ and N₂ are controlled to be equal to the voltage V_(C) applied to the positive input terminals in the respective amplifier circuits A₁ and A₂.

Meanwhile, the current consumed in the operation of the amplifier circuits A₁ and A₂ is smaller than the bias current received in the current amplifier circuit. Therefore, in the current amplifier circuit according to the present working example, the ratio between the bias current and the signal current can be improved, compared to the current amplifier circuit in the related art.

(Second Working Example)

FIG. 3 is a diagram illustrating a second working example according to the present embodiment. According to the present working example, the resistors R₁ and R₂ are the sensor devices that change the resistance values in the opposite sides, centering a predetermined resistance value R_(C) (R₁=R_(C)(1−Δ_(R)), and R₂=R_(C)(1+Δ_(R))). Further, the control circuit is connected to the current sources I₁ and I₂ at the nodes N₁ and N₂, and receives the bias current (I₁=I_(b) and I₂=I_(b)). The configurations of other components are same as the first working example. In other words, according to the present working example, the sensor devices are disposed inside the current amplifier circuit and the input signals are generated inside the current amplifier circuit.

According to the present working example, when the current I₁ and current I₂ flow in the resistor R₁ and R₂, the voltage at the node N₃ becomes: V₃=V_(C)+I_(b)R_(C)(1−Δ_(R)). The voltage at the node N₄ becomes: V₄=V_(C)+I_(b)R_(C)(1+Δ_(R)). Therefore, the current I_(Z1) flowing in the passive element Z₁ becomes: I_(Z1)=−2I_(b)R_(C)Δ_(R)/Z₁. Due to this, the current I_(M1) flowing in the transistor M₁ becomes: I_(M1)=I_(b)−2R_(C)I_(b)Δ_(R)/Z₁. The current I_(M2) flowing in the transistor M₂ becomes: I_(M2)=I_(b)+2R_(C)I_(b)Δ_(R)/Z₁.

With this configuration, the bias current I_(B) in the entire current amplifier circuit becomes: I_(B)=2I_(b). And, the signal current I_(SIG) to be output becomes: I_(SIG)=4I_(b)R_(C)Δ_(R)/Z₁. In other words, according to the present working example, the signal current can be amplified without amplifying the bias current in the current amplifier circuit as described above. Therefore, the ratio between the bias current and the signal current can be improved, compared to the current amplifier circuit in the related art.

Meanwhile, either the resistor R₁ or R₂ may be the fixed resistor having the resistance value R_(C). In this case, the signal current is generated at only one of the resistors R₁ and R₂, and the bias current I_(B) becomes: I_(B)=2I_(b). And, the signal current I_(SIG) becomes: I_(SIG)=−2I_(b)R_(C)Δ_(R)/Z₁.

(Third Working Example)

FIG. 4 is a diagram illustrating a third working example according to the present embodiment. The present working example is a modified example of the first working example, and the current amplifier circuit includes resistors R₃ and R₄ instead of the current sources I₁ and I₂ of the first embodiment. The resistor R₃ and R₄ are the sensor devices that change the resistance values in the opposite direction, centering the predetermined resistance value R_(C) (R₃=R_(C)(1+Δ_(R)), R₄=R_(C)(1−Δ_(R))) and the resistor R₃ and R₄ are respectively connected at the node N₁ and N₂ respectively. According to the present working example, the current I₁ and I₂ generated at the resistor R₃ and R₄ are to be the input signals.

According to the present working example, the voltage at the nodes N₁ and N₂ is controlled at the predetermined voltage V_(C), and therefore the current I₁ flowing in the resistor R₃ is: I₁=V_(C)/{R_(C)(1+Δ_(R)}, and the current I₂ flowing in the resistor R₄ is I₂=V_(C)/{R_(C)(1−Δ_(R))}. In the case of 1>>Δ_(R), the current I₁ and I₂ can be approximated as follows: I₁≦V_(C)/R_(C)−V_(C)Δ_(R)/R_(C), I₂≈V_(C)/R_(C)+V_(C)Δ_(R)/R_(C). More specifically, the current amplifier circuit according to the present working example differentially receives, from the nodes N₁ and N₂, the input signal on which the signal current (V_(C)Δ_(R)/R_(C)) is superimposed on the bias current (V_(C)/R_(C)). Therefore, the current amplifier circuit according to the present working example operates in the same manner as the first working example, and the ratio between the bias current and the signal current can be improved, compared to the current amplifier circuit in the related art.

(Fourth Working Example)

FIG. 5 is a diagram illustrating a fourth working example according to the present embodiment. The current amplifier circuit according to the present working example is formed by combining the current amplifier circuit of the second working example with that of the third working example, and all of the resistors R₁ to R₄ are sensor devices. The resistor R₁ has opposite polarity of resistance value change with respect to the resistor R₂, and the resistor R₃ has the opposite polarity of the resistance value change with respect to the resistor R₄. More specifically, the resistors are: R₁=R₄=R_(C)(1−Δ_(R)), and R₂=R₃=R_(C)(1+Δ_(R)).

In this case, as described in the third working example, the input signal is: I₁≈V_(C)/R_(C)−V_(C)Δ_(R)/R_(C), and I₂≈V_(C)/R_(C)+V_(C)Δ_(R)/R_(C). Therefore, the voltage V₃ at the node N₃ can be approximated as next: V₃≈V_(C)+I₁R₁=2V_(C)−2V_(C)Δ_(R)+V_(C)(Δ_(R))²≈2V_(C)−2V_(C)Δ_(R). Further, the voltage V₄ at the node N₄ can be approximated as next: V₄≈V_(C)+I₂R₂=2V_(C)+2V_(C)Δ_(R)+V_(C)(Δ_(R))²≈2V_(C)+2V_(C)Δ_(R). Therefore, the current I_(Z1) flowing in the passive element Z₁ becomes: I_(Z1)≈4V_(C)Δ_(R)/Z₁. Here, provided that the bias current I_(b)=V_(C)/R_(C) and the signal current contained in the input signal is I₁−I₂=2V_(C)Δ_(R)/R_(C), the bias current and the signal current become as next: I_(B)=2I_(b), and I_(SIG)=2V_(C)Δ_(R)/R_(C)+8V_(C)Δ_(R)/Z₁. In other words, according to the present working example, the signal current is further amplified and amplification of the bias current is suppressed. Therefore, the ratio between the bias current and the signal current can be improved, compared to the current amplifier circuit in the related art.

(Fifth Working Example)

FIG. 6 is a diagram illustrating a fifth working example according to the present embodiment. The present working example is a concrete example of the fourth working example, in which each of the amplifier circuits A₁ and A₂ according to the fourth working example are formed of an amplifier circuit including a plurality of transistors.

The amplifier circuit A₁ (A₂) includes a transistor M_(A11) (M_(A21)), a transistor M_(A12) (M_(A22)), a transistor M_(A13) (M_(A23)), a transistor M_(A14) (M_(A24)), and a current source I_(bA1) (I_(bA2)). The transistors M_(A11), M_(A12), M_(A21), and M_(A22) are NMOS transistors, and the transistors M_(A13), M_(A14), M_(A23), and M_(A24) are PMOS transistors.

The transistor M_(A11) (M_(A21)) has a gate terminal from which the predetermined bias voltage V_(C) is received, and a source terminal connected to the current source I_(bA1) (I_(bA2)). The transistor M_(A12) (M_(A22)) has a gate terminal connected to the node N₁ (N₂), and a source terminal connected to the current source I_(bA1) (I_(bA2)). The transistor M_(A13) (M_(A23)) and the transistor M_(A14) (M_(A24)) form the current mirror circuit, and the transistor M_(A14) (M_(A24)) has a drain terminal connected the gate terminal of the transistor M₁ (M₂).

With this configuration, when the voltage V₁ (V₂) at the node N₁ (N₂) becomes higher than the bias voltage V_(C), the drain current at the transistor M_(A11) (M_(A21)) is decreased and the drain current is copied at the current mirror circuit. Then, the output of the amplifier circuit A₁ (A₂) is decreased, i.e., the voltage at the gate terminal of the transistor M₁ (M₂) is decreased and the voltage V₁ (V₂) is decreased. In the same manner, when the voltage V₁ (V₂) at the node N₁ (N₂) becomes lower than the bias voltage V_(C), the drain current at the transistor M_(A11) (M_(A21)) is increased and the drain current is copied at the current mirror circuit. Then, the output of the amplifier circuit A₁ (A₂) is increased, i.e., the voltage at the gate terminal of the transistor M₁ (M₂) is increased and the voltage V₁ (V₂) is increased. Therefore, the voltage at the gate terminal of the transistor M₁ (M₂) is controlled by the amplifier circuit A₁ (A₂) such that the voltage V₁ (V₂) at the node N₁ (N₂) becomes equal to the bias voltage V_(C).

(Sixth Working Example)

FIG. 7 is a diagram illustrating a sixth working example according to the present embodiment. The present working example is a modified example of the fifth working example, in which the amplifier circuits A₁ and A₂ of the fifth working example are formed of a single amplifier circuit. In other words, according to the present working example, the control circuit is formed of the single amplifier circuit.

The control circuit includes transistors M_(A121), M_(A122), and M_(A123) each having a source terminal connected to the current source I_(bA), and transistors M_(A124), M_(A125), and M_(A126) forming the current mirror circuit. The transistors M_(A121), M_(A122), and M_(A123) are NMOS transistors, and the transistors M_(A124), M_(A125), and M_(A126) are PMOS transistors.

The transistor M_(A123) has a gate terminal from which the bias voltage V_(C) is applied. The transistor M_(A121) has a gate terminal connected to the node N₁, and the transistor M_(A122) has a gate terminal connected to the node N₂. Further, transistor M_(A121) has a drain terminal connected to the gate terminal of the transistor M₁, and the transistor M_(A122) has a drain terminal connected to the gate terminal of the transistor M₂.

The operation of the amplifier circuit A₁ according to the fifth working example is implemented by the amplifier circuit including the transistors M_(A121), M_(A123), M_(A124), and M_(A126), and the operation of the amplifier circuit A₂ is implemented by the amplifier circuit including the transistors M_(A122), M_(A123), M_(A125), and M_(A126). In the control circuit according to the present working example, noise generated from the transistors M_(A123), M_(A126), and the current source I_(bA) is superimposed on a common-mode output signal and is not superimposed on a differential output signal. Therefore, the current amplifier circuit having low noise can be achieved. Additionally, since the control circuit is configured of one single amplifier circuit, thereby achieving to reduce the current consumed at the control circuit.

(Seventh Working Example)

FIG. 8 is a diagram illustrating a seventh working example according to the present embodiment. The present working example is a modified example of the fourth working example, and further includes the transistors M₃, M₄ and a current source I_(b) _(—) _(add). The transistor M₃ (M₄) is an NMOS transistor, having a gate terminal connected to the source terminal of the transistor M₁ (M₂) at a node N₅ (N₆), a source terminal connected to the current source I_(b) _(—) _(add), and a drain terminal connected to the output terminal T₃ (T₄).

With this configuration, the voltage V₃ at the node N₃ is applied to the gate terminal of the transistor M₃, and the voltage V₄ at the node N₄ is applied to the gate terminal of the transistor M₄. Therefore, when the voltage V₃ (V₄) becomes higher than voltage V₄ (V₃) due to the signal current, the drain current at the transistor M₃ (M₄) is increased and the drain current at the transistor M₄ (M₃) is decreased. Due to this, the signal current can be further amplified because it is possible to generate an output signal in which the drain current of the transistor M₃ (M₄) having been increased (decreased) corresponding to the signal current is added to the drain current of the transistor M₁ (M₂).

(Eighth Working Example)

FIG. 9 is a diagram illustrating an eighth working example according to the present embodiment. The present working example is a modified example of the fifth working example, and the resistors R₃ and R₄ are connected to the current source I₀. With this configuration, the input signals (current I₁ and I₂) generated at the resistor R₃ and R₄ can be set without depending on manufacturing variation of the resistors R₃ and R₄ or the bias voltage V_(C). For example, in the case where the current I_(o) to be input from the current source I₀ has a current value 2I_(b), the current I₁ and I₂ become: I₁=I_(b)(1−Δ_(R)), and I₂=I_(b)(1+Δ_(R)).

(Ninth Working Example)

FIG. 10 is a diagram illustrating a ninth working example according to the present embodiment. The present working example is a modified example of the first working example, and the control circuit includes an inverting amplifier circuit A₁ (A₂) formed of a transistor M_(A11) (M_(A21)) and a current source I_(bA1) (I_(bA2)). The transistor M_(A11) (M_(A21)) is an NMOS transistor, having a source terminal grounded, a gate terminal connected to the node N₁ (N₂), and a drain terminal connected to the current source I_(bA1) (I_(bA2)) and to the gate terminal of the transistor M₁ (M₂).

With this configuration, the voltage at the gate terminal of the transistor M₁ (M₂) is controlled such that the voltage at the gate terminal of the transistor M_(A11) (M_(A21)), namely, the voltage V₁ (V₂) at the node N₁ (N₂) becomes the gate-source voltage V_(GS) _(—) _(MA11) (V_(GS) _(—) _(MA21)) of the transistor M_(A11) (M_(A21)) determined in accordance with current at the current source I_(bA1) (I_(bA2)). Therefore, the voltage V₁ (V₂) can be equal to the gate-source voltage V_(GS) _(—) _(MA11) (V_(GS) _(—) _(MA21)) by equalizing the device size of the transistors M_(A11) and M_(A21) or the current values at the current sources I_(bA1) and I_(bA2). According to the present working example, the number of components in the control circuit can be reduced and the voltage V₁ (V₂) at the nodes N₁ and N₂ can be equalized with a more simple circuit.

(Tenth Working Example)

FIG. 11 is a diagram illustrating a tenth working example according to the present embodiment. The present working example is a modified example of the first working example, and further includes transistors M₃ and M₄ and output terminals T₅ and T₆. The transistor M₃ (M₄) is an NMOS transistor, having a drain terminal connected to the output terminal T₅ (T₆), a source terminal connected to the source terminal of the transistor M₁ (M₂) at the node N₅ (N₆), and a gate terminal connected to the control circuit. The voltage at the gate terminal of the transistor M₃ (M₄) is controlled by the control circuit.

With this configuration, the number of output terminals is increased and the larger number of multiple output signals can be output. A current value of the output signal output from each of the output terminals is determined by the device size ratio of each of the transistors. Note that the number of the transistors and the output terminals to be increased may be optionally selected, for example, same on the right side and left side of the passive element Z₁ or may be different in each side.

(Eleventh Working Example)

FIG. 12 is a diagram illustrating an eleventh working example according to the present embodiment. The present working example is a modified example of the first working example, and includes transistors M_(1a) (M_(2a)) and M_(1b) (M_(2b)) instead of the transistor M₁ (M₂). The amplifier circuit A₁ (A₂) forming the control circuit has the negative input terminal from which the bias voltage V_(C1) is applied, and has the positive input terminal connected to the node N₁ (N₂). The transistor M_(1a) (M_(2a)) is a PMOS transistor, having a drain terminal connected to the resistor R₁ (R₂) and to the passive element Z₁ at the node N₃ (N₄), a gate terminal connected to the control circuit, and a source terminal connected to a source terminal of the transistor M_(1b) (M_(2b)). The transistor M_(1b) (M_(2b)) is an NMOS transistor, having the source terminal connected to the source terminal of the transistor M_(1a) (M_(2a)), a gate terminal from which a predetermined bias voltage V_(C2) is applied, and a drain terminal connected to the output terminal T₃ (T₄).

With this configuration, when the voltage at the gate terminal of the transistor M_(1a) (M_(2a)) becomes high, the gate-source voltage at the transistor M_(1a) (M_(2a)) and transistor M_(1b) (M_(2b)) becomes low, thereby decreasing the current flowing in the transistor M_(1a) (M_(2a)) and transistor M_(1b) (M_(2b)). In other words, the polarity becomes opposite to the transistor M₁ (M₂) of the first working example. Therefore, the bias voltage V_(C1) is received from the negative input terminal of the amplifier circuit A₁ (A₂) and the positive input terminal is connected to the node N₁ (N₂), thereby forming a feedback circuit and achieving to control the voltage V₁ (V₂) at the node N₁ (N₂) to be the bias voltage V_(C1).

Further, according to the first working example, a gain from the gate terminal to the source terminal in the transistor M₁ (M₂) is substantially one; however, the gain from the gate terminal to the drain terminal in the transistor M_(1a) (M_(2a)) can be set at one or larger according to the present working example. Due to this, it is possible to make an open loop gain large and make a voltage difference between the positive input terminal and the negative input terminal of the amplifier circuit A₁ (A₂) small. Therefore, the control circuit according to the present working example is capable of accurately controlling the voltage at the gate terminal of the transistor M_(1a) (M_(2a)).

(Twelfth Working Example)

FIG. 13 is a diagram illustrating a twelfth working example according to the present embodiment. The present working example is a modified example of the eleventh working example, and further includes level shift circuits LS₁ and LS₂. The level shift circuit LS₁ (LS₂) is connected to the control circuit, the gate terminal of the transistor M_(1a) (M_(2a)) and the gate terminal of the transistor M_(2b) (M_(1b)). The level shift circuit LS₁ (LS₂) converts the output voltage of the amplifier circuit A₁ (A₂) so as to match an operating point of the transistor M_(2b) (M_(1b)), and applies the converted voltage to the gate terminal of the transistor M_(2b) (M_(1b)).

With this configuration, when the output voltage of the amplifier circuit A₁ (A₂), namely, the voltage at the gate terminal of the transistor M_(1a) (M_(2a)) becomes high, operation is executed such that the voltage at the gate terminal of the transistor M_(1b) (M_(2b)) becomes low. Due to this, the drain current at the transistor M_(1a) (M_(2b)) is further decreased, compared to the twelfth working example. Therefore, it is possible to make the gain from the gate terminal to the drain terminal at the transistor M_(1a) (M_(2a)) large and make the voltage difference between the positive input terminal and the negative input terminal at the amplifier circuit A₁ (A₂) small. As a result, the voltage at the gate terminal of the transistor M_(1a) (M_(2a)) can be accurately controlled.

(Thirteenth Working Example)

FIG. 14 is a diagram illustrating a thirteenth working example according to the present embodiment. The present working example is a concrete example of the twelfth working example, in which the level shift circuits LS₁ and LS₂ are formed of a plurality of the transistors inside the control circuit. The configuration of the control circuit is same as the eighth working example.

The level shift circuit LS₁ (LS₂) includes a transistor M_(LS11) (M_(LS21)) and a transistor M_(LS12) (M_(LS22)). The transistor M_(LS11) (M_(LS21)) is a PMOS transistor, having a drain terminal connected to the gate terminal of the transistor M_(1a) (M_(2a)) and to the drain terminal of the transistor M_(A11) (M_(A21)), a gate terminal diode-connected to the drain terminal, and a source terminal connected to a source terminal of the transistor M_(LS12) (M_(LS22)). The transistor M_(LS12) (M_(LS22)) is an NMOS transistor, having the source terminal connected to the source terminal of the transistor M_(LS11) (M_(LS21)), a drain terminal connected to the gate terminal of the transistor M_(2b) (M_(1b)) and to the drain terminal of the transistor M_(A13) (M_(A23)), and a gate terminal diode-connected to the drain terminal. Thus, the bias current between the level shift circuit LS₁ (LS₂) and the amplifier circuit A₁ (A₂) can be shared by disposing the level shift circuit LS₁ (LS₂) inside the control circuit (amplifier circuits A₁ and A₂). Therefore, increase of the bias current in the entire current amplifier circuit can be suppressed.

(Fourteenth Working Example)

FIG. 15 is a diagram illustrating a fourteenth working example according to the present embodiment. The present working example is a modified example of the fourth working example, in which the control circuit is formed of a single amplifier circuit A₁. The amplifier circuit A₁ has the negative input terminal connected to the resistor R₁ at the node N₁, and the positive input terminal connected to the resistor R₂ at the node N₂. Further, the transistor M₂ has the gate terminal from which the bias voltage V_(C) is applied. The amplifier circuit A₁ compares the voltage V₁ at the node N₁ with the voltage V₂ at the node N₂, and controls the voltage at the gate terminal of the transistor M₁ such that the voltage V₁ and V₂ become equal to a predetermined voltage set by the bias voltage V_(C). With this configuration, the number of components in the control circuit can be reduced.

(Fifteenth Working Example)

FIG. 16 is a diagram illustrating a fifteenth working example of the present embodiment. The present working example is a modified example of the eleventh working example, in which the control circuit is formed of the single amplifier circuit A₁, same as the fourteenth working example. In this case, the polarity of the transistor M_(1a) is opposite to the polarity of the transistor M₁ of the fourteenth working example. Therefore, the amplifier circuit A₁ has the positive input terminal connected to the resistor R₁ at the node N₁, and the negative input terminal connected to the resistor R₂ at the node N₂. With this configuration, the number of components in the control circuit can be reduced.

(Sixteenth Working Example)

FIG. 17 is a diagram illustrating a sixteenth working example of the present embodiment. The present working example is a modified example of the eleventh working example, in which the control circuit is formed of a common-gate circuit including transistors M₅ and M₆. The transistor M₅ (M₆) is a PMOS transistor, having a gate terminal from which the bias voltage V_(C1) is applied, a source terminal connected to the resistor R₁ (R₂) at the node N₁ (N₂), and a drain terminal connected to the gate terminal of the transistor M_(1a) (M_(2a)) and the current sources I₁ and I₂.

With this configuration, the input signals received from the current sources I₁ and I₂ are applied to the resistors R₁ and R₂ via the transistors M₅ and M₆. Further, the voltage at the source terminals of the transistors M₅ and M₆, namely, the voltage V₁ and V₂ at the nodes N₁ and N₂ become substantially equal in the case of Δ_(R)<<1, because the respective current are: I₁=I_(b)(1+Δ_(R)) and I₂=I_(b)(1−Δ_(R)). With this configuration, the amplifier circuit is not used as the control circuit, and therefore increase of the bias current can be suppressed.

(Second Embodiment)

Next, a current amplifier circuit according to a second embodiment will be described with reference to FIGS. 18 to 20. Here, FIG. 18 is a diagram illustrating the current amplifier circuit according to the second embodiment. As illustrated in FIG. 18, the current amplifier circuit according to the present embodiment includes a transistor M₁, a resistor R₁, a transistor M₂, a resistor R₂, a passive element Z₁, and a control circuit, and impedance element Z₁ of the passive element Z₁ is controlled by the control signal Cont. The configurations of other components are same as the first embodiment.

The passive element Z₁ according to the present embodiment may be formed of, for example, a plurality of resistors Rg₁ to Rg₃ connected in parallel and switches connected to the respective resistors in series, as illustrated in FIG. 19. The impedance element Z₁ (resistance value in this case) can be changed by controlling opening and closing of the respective switches by the control signal Cont. Additionally, there are other possible configurations in which a plurality of capacitor is connected in parallel instead of the resistors or the resistor and the capacitor are combined.

Also, as illustrated in FIG. 20, the passive element Z₁ may be configured by connecting a transistor M_(R) and a resistor Rg in parallel. By controlling voltage at a gate terminal of the transistor M_(R) with the control signal Cont, the passive element Z₁ can be variable resistor and the impedance element Z₁ can be controlled.

As described in the first embodiment, a signal current I_(SIG) is determined by the impedance element Z₁. Therefore, in the current amplifier circuit according to the present embodiment, an amplification factor of the signal current can be controlled by controlling the impedance element Z₁ of the passive element Z₁ by the control signal Cont.

(Third Embodiment)

Next, a current amplifier circuit according to a third embodiment will be described with reference to FIG. 21. Here, FIG. 21 is a diagram illustrating the current amplifier circuit according to the third embodiment. As illustrated in FIG. 21, the current amplifier circuit according to the present embodiment includes a transistor M₁, a resistor R₁, a transistor M₂, a resistor R₂, a passive element Z₁, and a control circuit. The configurations of the above components are same as the first embodiment. The current amplifier circuit according to the present embodiment further includes a transistor M₃, a transistor M₄, a passive element Z₂, and current sources I₃, I₄.

The transistor M₃ (third transistor) is a PMOS transistor, having a drain terminal (first terminal) connected to a drain terminal of the transistor M₁ and to an output terminal T₃, a gate terminal (control terminal) connected to a gate terminal of the transistor M₁ and to a control circuit, and a source terminal connected to the passive element Z₂ and to the current source I₃.

The transistor M₄ (fourth transistor) is the PMOS transistor, having a drain terminal (first terminal) connected to a drain terminal of the transistor M₂ and to an output terminal T₄, a gate terminal (control terminal) connected to a gate terminal of the transistor M₂ and to the control circuit, and a source terminal connected to the passive element Z₂ and to the current source I₄.

The passive element Z₂ (second passive element) is connected to a source terminal of the transistor M₃ and a source terminal of the transistor M₄. In the passive element Z₂, current I_(Z2) flows. Note that the configuration of the passive element Z₂ is same as the passive element Z₁.

The current source I₃ is connected between the source terminal of the transistor M₃ and a power source and supplies bias current I_(b) to the transistor M₃. The current source I₄ is connected between the source terminal of the transistor M₄ and the power source, and supplies the bias current I_(b) to the transistor M₄.

Next, operation of the current amplifier circuit according to the present embodiment will be described. When current I₁ (I₂) received from an input terminal T₁ (T₂) becomes larger, the control circuit controls the voltage at the gate terminal of the transistor M₁ (M₂) such that drain current I_(M1) (I_(M2)) at the transistor M₁ (M₂) becomes large. As a result, the voltage at the gate terminal of the transistor M₁ (M₂) becomes high. In this instance, voltage at the gate terminal of the transistor M₃ (M₄) also becomes high because the gate terminal of the transistor M₃ (M₄) is connected to the control circuit. Since the transistor M₃ (M₄) is the PMOS transistor, the drain current I_(M3) (I_(M4)) becomes small when the voltage at the gate terminal becomes high.

In the same manner, when the current I₁ (I₂) received from the input terminal T₁ (T₂) becomes small, the control circuit controls the voltage at the gate terminal of the transistor M₁ (M₂) such that drain current I_(M1) (I_(M2)) flowing in the transistor M₁ (M₂) becomes small. Therefore, the voltage at the gate terminal of the transistor M₁ (M₂) becomes low. Due to this, the voltage at the gate terminal of the transistor M₃ (M₄) also becomes low and the drain current I_(M3) (I_(M4)) becomes large. Thus, the transistor M₃ (M₄) complementarily operates with respect to the transistor M₁ (M₂).

Therefore, when the current I₁ and I₂ are received from the input terminals T₁ and T₂, a difference between the current I₁ and current I₂, namely, the current I_(Z2) corresponding to Δ_(R) flows in the passive element Z₂, and the drain current in the transistors M₃ and M₄ become: I_(M3)=I_(b)+I_(Z2), and I_(M4)=I_(b)−I_(Z2). Due to this, the signal current I_(SIG) contained in the output signal output from the output terminals T₃ and T₄ becomes a sum of I_(M1)−I_(M2) and I_(M3)−I_(M4).

As described above, in the current amplifier circuit according to the present embodiment, the signal current can be amplified larger than in the current amplifier circuit according to the first embodiment. Therefore, compared to the current amplifier circuit in the related art, the ratio between the bias current and the signal current can be improved.

(Fourth Embodiment)

Next, a current amplifier circuit according to a fourth embodiment will be described with reference to FIG. 22. Here, FIG. 22 is a diagram illustrating the current amplifier circuit according to the fourth embodiment. As illustrated in FIG. 22, the current amplifier circuit according to the present embodiment includes a transistor M₁, a resistor R₁, a transistor M₂, a resistor R₂, a passive element Z₁, a control circuit, a transistor M₃, a transistor M₄, a passive element Z₂, and current sources I₃ and I₄. According to the present embodiment, a gate terminal of the transistor M₃ (M₄) is connected to a source terminal of the transistor M₁ (M₂), and the configurations of other components are same as the third embodiment.

Next, operation of the current amplifier circuit according to the present embodiment will be described. Voltage V₁ (V₂) at a node N₁ (N₂) is controlled to be same by the control circuit, and therefore, when current I₁ (I₂) received from an input terminal T₁ (T₂) becomes larger, voltage V₃ (V₄) at a node N₃ (N₄) becomes large. A gate terminal of the transistor M₃ (M₄) is connected to a source terminal of the transistor M₁ (M₂), namely, the node N₃ (N₄), and therefore the voltage at the gate terminal of the transistor M₃ (M₄) becomes high. Since the transistor M₃ (M₄) is a PMOS transistor, drain current I_(M3) (I_(M4)) becomes small when the voltage at the gate terminal becomes high.

In the same manner, when the current I₁ (I₂) received from the input terminal T₁ (T₂) becomes small, the voltage V₃ (V₄) at the node N₃ (N₄) becomes low, and also the voltage at the gate terminal of the transistor M₃ (M₄) becomes low and the drain current I_(M3) (I_(M4)) becomes large. Thus, the transistor M₃ (M₄) complementarily operates with respect to the transistor M₁ (M₂). Therefore, in the current amplifier circuit according to the present embodiment, signal current I_(SIG) contained in an output signal output from the output terminals T₃ and T₄ is a sum of I_(M1)−I_(M2) and I_(M3)−I_(M4), and signal current can be amplified larger than the current amplifier circuit according to the first embodiment, same as the third embodiment. Meanwhile, according to the present embodiment, in the case where the current I₁ and I₂ are small and the transistors M₃ and M₄ operate in a saturation region, the current I_(M3) and I_(M4) change in accordance with Δ_(R). Therefore, the source terminals of the transistors M₃ and M₄ may be short-circuited (Z₂=0).

(Fifth Embodiment)

Next, a current amplifier circuit according to a fifth embodiment will be described with reference to FIGS. 23 to 25. Here, FIG. 23 is a diagram illustrating the current amplifier circuit according to the fifth embodiment. As illustrated in FIG. 23, the present embodiment is a modified example of the first embodiment, and includes a switching circuit whereby paths are switched such that current I_(M1) flowing in a transistor M₁ and current I_(M2) flowing in a transistor M₂ are alternately switched. The switching circuit includes a switch 1 and a switch 2.

The switch 1 (first switch) is a chopper circuit that switches a path between a path of the current I_(M1) output from a drain terminal of the transistor M₁ and a path of the current I_(M2) output from a drain terminal of the transistor M₂. The switch 1 is disposed between output terminals T₃, T₄ of the current amplifier circuit and input terminals T₅, T₆ of a post-stage circuit where output signals are received from the current amplifier circuit. The switch 1 alternately switches the path between the path in which the output terminal T₃ and the input terminal T₅ are connected and also the output terminal T₄ and the input terminal T₆ are connected, and the path in which the output terminal T₃ and the input terminal T₆ are connected and also the output terminal T₄ and input terminal T₅ are connected.

In the case where the output terminal T₃ and the input terminal T₅ are connected and the output terminal T₄ and the input terminal T₆ are connected by the switch 1, the current I_(M1) output from the output terminal T₃ is received in the input terminal T₅ and the current I_(M2) output from the output terminal T₄ is received in the input terminal T₆. When the switch 1 switches the path, the output terminal T₃ is connected to the input terminal T₆ and the output terminal T₄ is connected to the input terminal T₅. Due to this, the current I_(M1) output from the output terminal T₃ is received in the input terminal T₆, and the current I_(M2) output from the output terminal T₄ is received in the input terminal T₅.

The switch 2 (second switch) is a chopper circuit that switches the path between a path of current I₁ received from the other end of a resistor R₁, namely, a node N₁, and a path of current I₂ received from the other end of a resistor R₂, namely, a node N₂. The switch 2 is disposed between the input terminals T₁, T₂ and the nodes N₁, N₂ in the current amplifier circuit. The switch 2 alternately switches the path between the path in which the input terminal T₁ and the node N₁ are connected and also the input terminal T₂ and the node N₂ are connected, and the path in which the input terminal T₁ and the node N₂ are connected and also the input terminal T₂ and the node N₁ are connected.

In the case where the input terminal T₁ and the node N₁ are connected and the input terminal T₂ and the node N₂ are connected by the switch 2, the current I₁ received from the input terminal T₁ is received at the node N₁ via a control circuit and the current I₂ received from the input terminal T₂ is received at the node N₂ via the control circuit. When the switch 2 switches the path, the input terminal T₁ is connected to the node N₂ and the input terminal T₂ is connected to the node N₁. Due to this, the current I₁ received from the input terminal T₁ is received at the node N₂ via the control circuit, and the current I₂ received from the input terminal T₂ is received at the node N₁ via the control circuit.

Switching timings of the switch 1 and the switch 2 are synchronized. The switches 1 and 2 operate such that the switch 2 connects the input terminal T₁ (T₂) to the node N₁ (N₂) in the case where the switch 1 connects the output terminal T₃ (T₄) to the input terminal T₅ (T₆), and the switch 2 connects the input terminal T₁ (T₂) to the node N₂ (N₁) in the case where the switch 1 connects the output terminal T₃ (T₄) to the input terminal T₆ (T₅).

FIGS. 24A to 24F are diagrams for describing changes of a signal by the operation of the switching circuit. In the following, a description will be given for the case in which a signal component exists in a low-frequency region of an input signal as illustrated in FIG. 24A. The current amplifier circuit sometimes generates noise in the process of amplifying the input signal. Particularly, flicker noise (1/f noise) may be generated in the low-frequency region as illustrated in FIG. 24B. In the case where such noise is generated, the output signal of the current amplifier circuit becomes a signal in which a signal component and a noise component are superimposed in the low-frequency region, as illustrated in FIG. 24C.

On the other hand, in the case where the signal is amplified while the switching circuit switches the path at a predetermined frequency, the signal component contained in the input signal is shifted to a high-frequency region by an amount of frequency switched by the switch 2 as illustrated in FIG. 24D. The output signal becomes a signal in which the signal component shifted to the high-frequency region and the noise component in the above-mentioned low-frequency region are superimposed. Therefore, as illustrated in FIG. 24E, the signal component contained in the output signal and the noise component are not overlapped.

In the output signal, switching frequency is combined again by the switch 1. Here, in the case of setting phases of the switching frequency of the switch 1 and switch 2 inversed, the signal component contained in the output signal is returned to the low-frequency region, and the noise component is shifted to the high-frequency region, as illustrated in FIG. 24F. Therefore, in the current amplifier circuit according to the present embodiment, the noise component can be easily removed from the output signal by a filter such as a lowpass filter. With this configuration, influence of the noise generated in the current amplifier circuit can be suppressed.

(First Working Example)

FIG. 25 is a first working example of the present embodiment. The present working example is a concrete example of the fifth embodiment, where the switching circuit is applied in the current amplifier circuit of the first working example according to the first embodiment. With this configuration, the noise generated at the current amplifier circuit can be easily removed from the output signal.

(Sixth Embodiment)

Next, a current amplifier circuit according to a sixth embodiment will be described with reference to FIGS. 26 and 27. Here, FIG. 26 is a diagram illustrating the current amplifier circuit according to the sixth embodiment. As illustrated in FIG. 26, the present embodiment is a modified example of the fifth embodiment, and includes a switching circuit whereby paths are switched such that current I_(M1) flowing in a transistor M₁ and current I_(M2) flowing in a transistor M₂ are alternately switched. In the description of the present embodiment, a control circuit portion for controlling voltage at a gate terminal of the transistor M₁ is referred to as control circuit 1, and a control circuit portion for controlling voltage at a gate terminal of the transistor M₂ is referred to as control circuit 2. The configurations of other components are same as the first embodiment. The switching circuit includes a switch 1, a switch 3, a switch 4, and a switch 5. Note that description for the configuration of the switch 1 will be omitted here as is same as the fifth embodiment.

The switch 3 (third switch) is a chopper circuit that switches a path between one end of a resistor R₁, namely, a path of current I₁ output from the side of a node N₃ of the resistor R₁, and one end of a resistor R₂, namely, a path of current I₂ output from the side of a node N₄ of the resistor R₂. The switch 3 is disposed between the resistors R₁, R₂ and the nodes N₃, N₄. The switch 3 alternately switches the path between the path in which the resistor R₁ and the node N₃ are connected and also the resistor R₂ and the node N₄ are connected, and the path in which the resistor R₁ and the node N₄ are connected and also the resistor R₂ and the node N₃ are connected.

In the case where the resistor R₁ and the node N₃ are connected and the resistor R₂ and the node N₄ are connected by the switch 3, the current I₁ output from one end of the resistor R₁ is received at the node N₃ and the current I₂ output from one end of the resistor R₂ is received at the node N₄. When the switch 3 switches the path, the resistor R₁ is connected to the node N₄ and the resistor R₂ is connected to the node N₃. Due to this, the current I₁ output from one end of the resistor R₁ is received at the node N₄, and the current I₂ output from one end of the resistor R₂ is received at the node N₃.

The switch 4 (fourth switch) is a chopper circuit that switches the path for the current I₁ and I₂ received in the control circuit 1 in order to control the voltage at the gate terminal of the transistor M₁. The switch 4 is disposed between the input terminals T₁, T₂ and the control circuit 1. The switch 4 alternatively switches the path between the path in which the input terminal T₁ and the control circuit 1 are connected, and the path in which the input terminal T₂ and the control circuit 1 are connected.

In the case where the input terminal T₁ and the control circuit 1 are connected by the switch 4, the voltage at the input terminal T₁, namely the voltage at the other end of the resistor R₁ is received in the control circuit 1. When the switch 4 switches the path, the input terminal T₂ is connected to the control circuit 1 and the voltage at the input terminal T₂, namely the voltage at the other end of the resistor R₂ is received in the control circuit 1.

The switch 5 (fifth switch) is a chopper circuit that switches the path for the current I₁ and I₂ received in the control circuit 2 in order to control the voltage at the gate terminal of the transistor M₂. The switch 5 is disposed between the input terminals T₁, T₂ and the control circuit 2. The switch 5 alternatively switches the path between the path in which the input terminal T₁ and the control circuit 2 are connected, and the path in which the input terminal T₂ and the control circuit 2 are connected.

In the case where the input terminal T₁ and the control circuit 2 are connected by the switch 5, the voltage at the input terminal T₁, namely, the voltage at the other end of the resistor R₁ is received in the control circuit 2. When the switch 5 switches the path, the input terminal T₂ is connected to the control circuit 2 and the voltage at the input terminal T₂, namely, the voltage at the other end of the resistor R₂ is received in the control circuit 2.

Switching timings of the switches 1, 3, 4 and 5 are synchronized. In the case where the switch 1 connects an output terminal T₃ (T₄) to an input terminal T₅ (T₆), the switch 3 connects the resistor R₁ (R₂) to the node N₃ (N₄), the switch 4 connects the input terminal T₁ to the control circuit 1, and the switch 5 connects the input terminal T₂ to the control circuit 2. In this case, signal current of the current I₁ (I₂) received from the input terminal T₁ (T₂) is amplified and received in the input terminal T₅ (T₆). The control circuit 1 (control circuit 2) forms a negative feedback circuit together with the transistor M₁ (M₂) and the resistor R₁ (R₂), and controls the voltage at the gate terminal of the transistor M₁ (M₂) such that voltage at a node N₁ (N₂) becomes a predetermined voltage.

On the other hand, in the case where the switch 1 connects an output terminal T₃ (T₄) to an input terminal T₆ (T₅), the switch 3 connects the resistor R₁ (R₂) to the node N₄ (N₃), the switch 4 connects the input terminal T₂ to the control circuit 1, and the switch 5 connects the input terminal T₁ to the control circuit 2. In this case, the signal current of the current I₁ (I₂) received from the input terminal T₁ (T₂) is amplified and received in the input terminal T₅ (T₆). The control circuit 1 (control circuit 2) forms the negative feedback circuit together with the transistor M₁ (M₂) and the resistor R₂ (R₁), and controls the voltage at the gate terminal of the transistor M₁ (M₂) such that voltage at a node N₂ (N₁) becomes the predetermined voltage.

With the above-described structure, in the current amplifier circuit according to the present embodiment, noise component can be easily removed from the output signal by a filter such as a lowpass filter in the same manner as the fifth embodiment. Due to this, influence of the noise generated in the current amplifier circuit can be suppressed.

(First Working Example)

FIG. 27 is a diagram illustrating a first working example of the present embodiment. The present working example is a concrete example of the present embodiment, where the switching circuit is applied in a fourth working example of the first embodiment. In the present working example, the control circuit 1 is an amplifier circuit A₁, and the control circuit 2 is an amplifier circuit A₂. With this configuration, the noise generated in the current amplifier circuit can be easily removed from the output signal.

(Seventh Embodiment)

Next, a current amplifier circuit according to a seventh embodiment will be described with reference to FIGS. 28 to 30. Here, FIG. 28 is a diagram illustrating the current amplifier circuit according to the seventh embodiment. As illustrated in FIG. 28, the current amplifier circuit according to the present embodiment includes a transistor M₁, a resistor R₁, a passive element Z₁, and a control circuit. In other words, the current amplifier circuit according to the present embodiment is formed of a current amplifier circuit according to the above-described embodiments in a single-ended configuration.

The transistor M₁ (first transistor) is an NMOS transistor having a source terminal (first terminal) connected to the resistor R₁ and the passive element Z₁ at a node N₃, a drain terminal (second terminal) connected to an output terminal T₃, and a gate terminal (control terminal) connected to the control circuit. Voltage at the gate terminal of the transistor M₁ is controlled by the control circuit, and drain current I_(M1) flows in the transistor M₁.

The resistor R₁ (first resistor) has one end connected to the source terminal of the transistor M₁ and to the passive element Z₁ at the node N₃, and the other end connected to the control circuit at a node N₁. The resistor R₁ may be a fixed resistor having a constant resistance value, or may be a sensor device that detects a state of an object by change of the resistance value.

The passive element Z₁ (first passive element) has one end connected to the source terminal of the transistor M₁ and to one end of the resistor R₁ at the node N₃, and the other end connected to the ground (reference potential). The passive element Z₁ is formed of at least a capacitor, and an operating point of the current amplifier circuit is fixed by this capacitor. The passive element Z₁ may include resistor.

The control circuit is connected to the gate terminal of the transistor M₁, the other end of the resistor R₁, and an input terminal T₁. The control circuit controls the voltage at the gate terminal of the transistor M₁ such that voltage at the other end of the resistor R₁ becomes equal to a predetermined voltage (first predetermined voltage), namely, such that voltage V₁ at the node N₁ becomes equal to the predetermined voltage. Further, the control circuit executes control such that current I₁ received from the input terminal T₁ is applied to the resistor R₁.

Next, operation of the current amplifier circuit according to the present embodiment will be described. In the following description, the resistor R₁ is a fixed resistor: R₁=R_(C). The voltage V₁ at the node N₁ is controlled by the control circuit to be a voltage value V_(C) (V₁=V_(C)). Also, the current I₁ is: I₁=I_(b) (1+Δ_(R)). More specifically, a description will be given for a case in which the current amplifier circuit is connected to a sensor device, and signal current is received from the input terminal T₁.

When the current I₁ is received from the input terminal T₁, the voltage V₃ at the node N₃ is: V₃=V₁+R₁I₁. When following values R₁=R_(C), I₁=I_(b)(1+Δ_(R)), and V₁=V_(C) are plugged into the above formula, the voltage V₃ becomes: V₃=V_(C)+R₁I_(b)+R₁I_(b)Δ_(R).

Since the passive element Z₁ is connected between the node N₃ and the ground, the voltage V₃ at the node N₃ is applied. The passive element Z₁ includes the capacitor, and therefore direct current does not flow. For this reason, following current I_(Z1) corresponding to an AC component of the voltage V₃, namely, R₁I_(b)Δ_(R) flows in the passive element Z₁, and the current I_(Z1) is: I_(Z1)=R₁I_(b)Δ_(R)/Z₁. Due to this, the current I_(M1) flowing in the transistor M₁ becomes: I_(M1)=I₁+I_(Z1)=I_(b)+Δ_(R)I_(b), +R_(C)I_(b)Δ_(R)/Z₁. According to the present embodiment, the current I_(M1) becomes an output signal output from the output terminal T₃.

Therefore, bias current I_(B) flowing in the entire circuit becomes: I_(B)=I_(b). And, signal current I_(SIG) contained in the output signal becomes: I_(SIG)=Δ_(R)I_(b)+R_(C)I_(b)Δ_(R)/Z₁. As described above, the signal current is amplified and further amplification of the bias current can be suppressed in the current amplifier circuit according to the present embodiment, same as the above-described embodiment. Due to this, the ratio between the bias current and the signal current can be improved and power consumption in the current amplifier circuit can be suppressed, compared to the current amplifier circuit according to the related art.

(First Working Example)

FIG. 29 is a first working example of the present embodiment. According to the present working example, the current amplifier circuit includes an amplifier circuit A₁ as a control circuit. The amplifier circuit A₁ has a positive input terminal from which predetermined bias voltage V_(C) is applied, a negative input terminal connected to the other end of the resistor R₁ at the node N₁, and an output terminal connected to the gate terminal of the transistor M₁. Further, a resistor R₃ is connected to the input terminal T₁, and current I₁R₃ generated at the resistor R₃ becomes an input signal. The resistors R₁ and R₃ are sensor devices having following resistance values that change in the opposite direction, centering a predetermined resistance value R_(C): R₁=R_(C)(1−Δ_(R)) and R₃=R_(C)(1+Δ_(R)).

In the following, operation of the control circuit (amplifier circuit A₁) according to the present working example will be described. The amplifier circuit A₁ forms a feedback circuit together with the transistor M₁ and the resistor R₁, and executes feedback control such that the voltage at the positive and negative terminals of the amplifier circuit A₁ become equal. In other words, the amplifier circuit A₁ controls the voltage at the gate terminal of the transistor M₁ such that the voltage at the negative input terminal (voltage V₁ at the node N₁) becomes equal to the bias voltage V_(C). With this configuration, the voltage V₁ at the node N₁ is controlled to be equal to the bias voltage V_(C) applied to the positive input terminal of the amplifier circuit A₁.

Note that the current consumed by the operation of the amplifier circuit A₁ is smaller than the bias current received in the current amplifier circuit. Therefore, the ratio between the bias current and the signal current can be improved in the current amplifier circuit according to the present working example, compared to the current amplifier circuit according to the related art.

(Second Working Example)

FIG. 30 is a second working example of the present embodiment. The present working example is a concrete example of the first working example, and formed of an amplifier circuit in which the amplifier circuit A₁ of the first working example includes a plurality of transistors. Further, the passive element Z₁ includes the capacitor and the resistor. The amplifier circuit A₁ includes a transistor M_(A11), a transistor M_(A12), a transistor M_(A13), a transistor M_(A14), and a current source I_(bA1). The transistors M_(A11) and M_(A12) are NMOS transistors and the transistors M_(A13) and M_(A14) are PMOS transistors.

The transistor M_(A11) has a gate terminal from which the predetermined bias voltage V_(C) is applied, and a source terminal connected to the current source I_(bA1). The transistor M_(A12) has a gate terminal connected to the node N₁, and a source terminal connected to the current source I_(bA1). The transistor M_(A13) and the transistor M_(A14) form a current mirror circuit, and a drain terminal of the transistor M_(A14) is connected to the gate terminal of the transistor M₁. With this configuration, the voltage at the transistor M₁ is controlled by the amplifier circuit A₁ such that the voltage V₁ at the node N₁ becomes equal to the bias voltage V_(C).

(Eighth Embodiment)

Next, a current amplifier circuit according to an eighth embodiment will be described with reference to FIGS. 31 and 32. Here, FIG. 31 is a diagram illustrating the current amplifier circuit according to the eighth embodiment. As illustrated in FIG. 31, the current amplifier circuit according to the present embodiment includes a transistor M₁, a resistor R₁, a passive element Z₁, and a control circuit, and impedance element Z₁ of the passive element Z₁ is controlled by a control signal Cont. The configurations of other components are same as the seventh embodiment.

The passive element Z₁ according to the present embodiment may include, for example, a plurality of resistors connected in parallel, switches connected to the respective resistors in series, and a single capacitor connected to the respective resistors in series, as illustrated in FIG. 32. The impedance element Z₁ can be changed by controlling opening and closing of each switch with the control signal Cont. In the current amplifier circuit according to the present embodiment, an amplification factor of the signal current can be controlled by controlling the impedance element Z₁ of the passive element Z₁ with the control signal Cont. Meanwhile, the passive element Z₁ may include a plurality of capacitor connected in series to each of the resistors. Frequency characteristics of the passive element Z₁ can be optionally set by adjusting time constant of the resistor and the capacitor connected in series. For example, the frequency characteristics of the passive element Z₁ can be set constant by matching the time constant before and after switching of the switch.

(Ninth Embodiment)

Next, an integrator according to a ninth embodiment will be described with reference to FIGS. 33 to 39. Here, FIG. 33 is a diagram illustrating the integrator according to the ninth embodiment. As illustrated in FIG. 33, the integrator according to the present embodiment includes a current amplifier circuit according to the above-described embodiments and an integrating element that integrates an output signal of the current amplifier circuit, and includes a transistor M₁, a resistor R₁, a transistor M₂, a resistor R₂, a passive element Z₁, and a control circuit. The configurations of the above components are same as a current amplifier circuit according to the first embodiment.

The integrator according to the present embodiment further includes a capacitor element C connected to output terminals T₃ and T₄. The capacitor element C is formed including at least the capacitor. The capacitor element C integrates the output signal from the output terminals T₃ and T₄. The integrator can be easily created by the capacitor element C because the output signal of the current amplifier circuit is current.

(First Working Example)

FIG. 34 is a first working example of the present embodiment. The present working example is a modified example of the first working example according to the first embodiment, and includes a capacitor C₀ as the capacitor element C and further includes current sources I₃ and I₄. The capacitor C₀ is connected to the output terminals T₃ and T₄, and integrates the output signals. Current I₃ and I₄ of the current sources I₃ and I₄ are: I₃=I₄=I_(b). Therefore, only signal current I_(SIG) contained in the output signal can be integrated by the capacitor C_(O).

(Second Working Example)

FIG. 35 is a second working example of the present embodiment. The present working example is a modified example of the first working example, and includes the capacitor C₀ and a resistor R_(C) connected to the capacitor C₀ in series. With this configuration, phase compensation can be executed by inserting a zero point into the frequency characteristics. In this manner, the operation can be stabilized.

(Third Working Example)

FIG. 36 is a third working example of the present embodiment. The present working example is a modified example of the first working example, and includes capacitor C₁ and C₂ as the capacitor element C. The capacitor C₁ (first capacitor) is connected between ground and the output terminal T₃, namely, between the ground and a drain terminal of the transistor M₁. The capacitor C₂ (second capacitor) is connected between the ground and the output terminal T₄, namely, between the ground and a drain terminal of the transistor M₂. In the capacitor C₁ and C₂, the signal current contained in the output signal from the output terminals T₃ and T₄ is integrated. Meanwhile, in the case of using the integrator according to the present working example as a filter, the resistor may be connected to the capacitor C₁ and C₂ in series, same as the second working example. Due to this, the zero is introduced into the frequency characteristics, thereby achieving to perform the phase compensation and stabilize the operation.

(Fourth Working Example)

FIG. 37 is a fourth working example of the present embodiment. The present working example is a modified example of the third embodiment, and includes the capacitor C₀ connected between the output terminals T₃ and T₄ as the capacitor element C. According to the present working example also, only the signal current I_(SIS) contained in the output signal can be integrated by the capacitor C_(O).

(Fifth Working Example)

FIG. 38 is a fifth working example of the present embodiment. The present working example is a modified example of the fourth embodiment, and includes the capacitor C₁ and C₂ as the capacitor element, and further includes an amplifier circuit A_(integ). The amplifier circuit A_(integ) has differential inputs and differential outputs, and has a positive input terminal connected to the output terminal T₃ and a negative input terminal connected to the output terminal T₄. The capacitor C₁ (C₂) is connected between the positive input terminal T₃ (negative input terminal T₄) and the negative output terminal T₇ (positive output terminal T₈) of the amplifier circuit A_(integ). With this configuration, the input terminals of the amplifier circuit A_(integ) are virtually shorted, and the voltage between input terminals becomes equal. Therefore, even when an output voltage range of the current amplifier circuit is narrow, the output signal can be integrated.

(Sixth Working Example)

FIG. 39 is a diagram illustrating a sixth working example of the present embodiment. The present working example is a modified example of the first working example of the seventh embodiment, and includes the capacitor C₁ and C₂ as the capacitor element, and further includes the amplifier circuit A_(integ). The configurations of the capacitor C₁, C₂ and the amplifier circuit A_(integ) are same as the fifth working example. With this configuration, the input terminals of the amplifier circuit A_(integ) are virtually shorted, and the voltage between the input terminals of the amplifier circuit A_(integ) becomes equal. Therefore, even when an output voltage range of the current amplifier circuit is narrow, the output signal can be integrated. Further, the voltage between input and output of a switch 1, namely, between the output terminals T₃, T₄ and the input terminals T₅ and T₆ becomes small, and therefore, in the case of implementing the switch 1 in a MOS transistor, it is possible to suppress distortion of a signal caused by fluctuation of ON-resistance of the switch.

(Tenth Embodiment)

Next, an AD converter according to a tenth embodiment will be described with reference to FIGS. 40 to 42. Here, FIG. 40 is a diagram illustrating the AD converter according to the tenth embodiment. As illustrated in FIG. 40, the AD converter according to the present embodiment includes the current amplifier according to the above-described embodiments, a current-voltage conversion circuit, and an AD conversion circuit ADC. In FIG. 40, the configuration of the current amplifier is same as the first embodiment.

The current-voltage conversion circuit is connected to output terminals T₃ and T₄ of the current amplifier circuit and the AD conversion circuit ADC. The current-voltage conversion circuit converts differential signal current (output signal) output from the current amplifier circuit to differential signal voltage, and the differential signal voltage is received in the AD conversion circuit ADC. Meanwhile, in the case where the input signal of the AD conversion circuit ADC is current, the current-voltage conversion circuit is unnecessary. In this case, for example, the output terminals T₃ and T₄ of the current amplifier circuit may be directly connected to the AD conversion circuit ADC.

The AD conversion circuit ADC converts the differential signal voltage received via the current-voltage conversion circuit to a digital signal. This enables a state change of an object detected by a sensor to be output as the digital signal. In the AD converter according to the present embodiment, the AD conversion circuit ADC can be optionally selected. For example, it is possible to use a pipeline AD conversion circuit, a successive-approximation type AD conversion circuit, a single slope AD conversion circuit, a discrete time ΔΣAD conversion circuit, a continuous time ΔΣAD conversion circuit, and so on.

(First Working Example)

FIG. 41 is a diagram illustrating a first working example of the AD converter according to the present embodiment. The present working example is a concrete example of the AD converter in FIG. 40, and the current-voltage converter includes current sources I₃, I₄ and a resistor R_(V). The current sources I₃ and I₄ apply bias current I_(b), and the resistor R_(V) is connected between the output terminals T₃ and T₄. The differential signal current flows in the resistor R_(V), thereby generating voltage at both ends of the resistor R_(V), and the generated voltage is received by the AD conversion circuit ADC as the differential signal voltage.

(Second Working Example)

FIG. 42 is a diagram illustrating a second working example of the AD converter according to the present embodiment. The present working example is a modified example of the first working example, and the current-voltage converter includes resistors R_(V1), R_(V2) and transistors M_(V1), M_(V2). The resistors R_(V1) and R_(V2) are connected between the output terminals T₃ and T₄ in series. The transistor M_(V1) (M_(V2)) is a PMOS transistor, having a gate terminal mutually connected with a gate terminal of the other transistor, a source terminal connected to a power source, and a drain terminal connected to the output terminals T₃ (T₄). The resistors R_(V1) and R_(V2) have nodes connected to nodes of the transistors M_(V1) and M_(V2). With this configuration, the output signal of the current amplifier circuit can be converted to the voltage.

(Eleventh Embodiment)

Next, a current amplifier circuit according to an eleventh embodiment will be described with reference to FIGS. 43 and 44. Here, FIG. 43 is a diagram illustrating the current amplifier circuit according to the eleventh embodiment. As illustrated in FIG. 43, the current amplifier circuit according to the present embodiment includes a transistor M₁, a resistor R₁, a transistor M₂, a resistor R₂, a passive element Z₁, and a control circuit. The configurations of the above components are same as the first embodiment. The current amplifier circuit according to the present embodiment further includes a transistor M_(C1) and a transistor M_(C2).

The transistor M_(C1) (fifth transistor) is an NMOS transistor, having a source terminal (first terminal) connected to an input terminal T₁ and a drain terminal (second terminal) connected to the control circuit and the other end of the resistor R₁ at a node N₁, and applied with a predetermined bias voltage V_(c2) (third predetermined voltage) from a gate terminal (control terminal). In other words, the transistor M_(C1) forms a common-gate circuit.

The transistor M_(C2) (sixth transistor) is an NMOS transistor, having a source terminal (first terminal) connected to an input terminal T₂ and a drain terminal (second terminal) connected to the control circuit and the other end of the resistor R₂ at the node N₂, and applied with the predetermined bias voltage V_(c2) from a gate terminal (control terminal). In other words, the transistor M_(c2) forms a common-gate circuit.

With this configuration, the current amplifier circuit according to the present embodiment can suppress influence of the noise of the control circuit onto the output signal. The reason is as follows.

In a case where the control circuit and the input terminals T₁ and T₂ are directly connected as illustrated in the current amplifier circuit according to the first embodiment, a noise component is superimposed on current I₁ and I₂ input from the input terminals T₁ and T₂ respectively due to the influence of the noise of the control circuit. Then, the noise component contained in the current I₁ and I₂ is converted by the resistors R₁ and R₂ respectively to the voltage, converted by the passive element Z₁ to the current, and superimposed on the output signal. For this reason, for the use of low noise to be required, the control circuit for the low noise is necessary.

On the other hand, in the current amplifier circuit according to the present embodiment, the transistors M_(c1) and M_(c2) are respectively connected between the control circuit and the input terminals T₁ and T₂, the impedance when the input terminals T₁ and T₂ are viewed from the control circuit becomes high, the noise component contained in the current I₁ and I₂ can be reduced. Therefore, influence of the noise of the control circuit on the output signal can be suppressed.

(First Working Example)

FIG. 44 is a diagram illustrating a first working example of the present embodiment. The present working example is a concrete example of the present embodiment, where the transistors M_(c1) and M_(c2) are provided in the third working example (see FIG. 4) according to the first embodiment.

As described in the present working example, in a case where the transistor M_(c1) is provided between an amplifier circuit A₁ and a resistor R₃, an input conversion noise V_(n1) of the amplifier circuit A₁ is applied to the node N₁, the impedance of the input terminal T₁ viewed from the amplifier circuit A₁ becomes R₃×gm_(Mc1)×ro_(Mc1). Here, gm_(Mc1) is the transconductance of the transistor M_(c1), and ro_(Mc1) is the output resistance of the transistor M_(c1). In the case of 1<<gm_(Mc)×ro_(Mc1), the voltage (noise voltage) according to the noise component applied to the source terminal of the transistor M₁ (the node N₃) becomes V_(n1)+V_(n1)×R₁/(R₃×gm_(Mc1)×ro_(Mc1))≈V_(n1).

On the other hand, as described in the third working example of the first embodiment, in a case where the amplifier circuit A₁ and the resistor R₃ are directly connected, the input conversion noise V_(n1) of the amplifier circuit A₁ is applied to the node N₁, the impedance of the input terminal T₁ viewed from the amplifier circuit A₁ becomes R₃. Therefore, the noise component of V_(n1)/R₃ is superimposed on the current I₁, and the noise component is converted into a voltage by the resistor R₁. As a result, the noise voltage applied to the source terminal of the transistor M₁ (the node N₃) becomes V_(n1)+V_(n1)×R₁/R₃=V_(n1)(R₁+R₃)/R₃.

Thus, the noise voltage applied to the node N₃ can be reduced by disposing the transistor M_(c1) compared to the case where the amplifier circuit A₁ and the resistor R₃ are directly connected. The same thing is applied for the transistor M_(c2). Therefore, as described above, influence of the noise of the control circuit onto the output signal can be suppressed.

(Twelfth Embodiment)

Next, a current amplifier circuit according to a twelfth embodiment will be described with reference to FIGS. 45 and 46. Here, FIG. 45 is a diagram illustrating the current amplifier circuit according to the twelfth embodiment. As illustrated in FIG. 45, the current amplifier circuit according to the present embodiment includes a transistor M₁, a resistor R₁, a passive element Z₁, and a control circuit. The configurations of the above components are same as the seventh embodiment. The current amplifier circuit according to the present embodiment further includes a transistor M_(C1).

The transistor M_(C1) (second transistor) is an NMOS transistor, having a source terminal (first terminal) connected to an input terminal T₁ and a drain terminal (second terminal) connected to the control circuit and the other end of the resistor R₁ at a node N₁, and a predetermined bias voltage V_(c2) (second predetermined voltage) is input from a gate terminal (control terminal). In other words, the transistor M_(c1) forms a common-gate circuit.

With this configuration, the current amplifier circuit according to the present embodiment can suppress influence of the noise of the control circuit onto the output signal.

(First Working Example)

FIG. 46 is a diagram illustrating a first working example of the present embodiment. The present working example is a concrete example of the present embodiment, where the transistor M_(c1) is provided in the first working example (see FIG. 29) of the seventh embodiment.

As described in the present working example, in a case where the transistor M_(c1) is provided between an amplifier circuit A₁ and a resistor R₃, an input conversion noise V_(n1) of the amplifier circuit A₁ is applied to the node N₁, the impedance of the input terminal T₁ viewed from the amplifier circuit A₁ becomes R₃×gm_(Mc1)×ro_(Mc1). Here, gm_(Mc1) is the transconductance of the transistor M_(c1), and ro_(Mc1) is the output resistance of the transistor M_(c1). In the case of 1<<gm_(Mc1)×ro_(Mc1), the voltage (noise voltage) according to the noise component applied to the source terminal of the transistor M₁ (the node N₃) becomes V_(n1)+V_(n1)×R₁/(R₃×gm_(Mc1)×ro_(Mc1))≈V_(n1).

On the other hand, as described in the first working example of the seventh embodiment, in a case where the amplifier circuit A₁ and the resistor R₃ are directly connected, the input conversion noise V_(n1) of the amplifier circuit A₁ is applied to the node N₁, the impedance of the input terminal T₁ viewed from the amplifier circuit A₁ becomes R₃. Therefore, the noise component of V_(n1)/R₃ is superimposed on the current I₁, and the noise component is converted into a voltage by the resistor R₁. As a result, the noise voltage applied to the source terminal of the transistor M₁ (the node N₃) becomes V_(n1)+V_(n1)×R₁/R₃=V_(n1) (R₁+R₃)/R₃.

Thus, the noise voltage applied to the node N₃ can be reduced by disposing the transistor M_(c1) compared to the case where the amplifier circuit A₁ and the resistor R₃ are directly connected. Therefore, as described above, influence of the noise of the control circuit onto the output signal can be suppressed.

(Thirteenth Embodiment)

Next, a current amplifier circuit according to a thirteenth embodiment will be described with reference to FIGS. 47 and 48. Here, FIG. 47 is a diagram illustrating the current amplifier circuit according to the thirteenth embodiment. As illustrated in FIG. 47, the current amplifier circuit according to the present embodiment includes a transistor M₁, a resistor R₁, a transistor M₂, a resistor R₂, and a control circuit. The configurations of the above components are same as the first embodiment. In the present embodiment, the configuration of the passive element Z₁ is different from the first embodiment. In addition, a voltage-current conversion circuit Gm is connected to the passive element Z₁.

The passive element Z₁ includes a resistor RZ₁₁ (third resistor), a capacitor CZ₁, and a resistor RZ₁₂ (fourth resistor). One end of the resistor RZ₁₁ is connected to one end of the resistor R₁ and the source terminal of the transistor M₁ at the node N₃, and the other end is connected to one end of the capacitor CZ₁. One end of the resistor RZ₁₂ is connected to one end of the resistor R₂ and the source terminal of the transistor M₂ at the node N₄, and the other end is connected to the other end of the capacitor CZ₁. In other words, the resistor RZ₁₁, the capacitor CZ₁, and the resistor RZ₁₂ are connected in series.

The voltage-current conversion circuit Gm includes two input terminals InP (first input terminal) and InM (second input terminal) and two output terminals OutP (first output terminal) and OutM (second output terminal). The voltage-current conversion circuit Gm converts voltages input from the input terminals InP and InM into currents by a predetermined voltage-current conversion coefficient Gm, and outputs the converted currents to the output terminals OutP and OutM, respectively.

The input terminal InP of the voltage-current conversion circuit Gm is connected to the other end of the resistor RZ₁₁, the output terminal OutP is connected to one end of the resistor RZ₁₁, the input terminal InM is connected to the other end of the resistor RZ₁₂, and the output terminal OutM is connected to one end of the resistor RZ₁₂.

In the present embodiment, a voltage V₃ at the node N₃ becomes V₃=V₁+R₁I₁, and a voltage V₄ at the node N₄ becomes V₄=V₂+R₂I₂. Here, for example, in the case where R₁=R₂=R_(C), I₁=I_(b1)(1+Δ_(R)), I₂=I_(b2)(1−Δ_(R)), and V₁=V₂=V_(C), the voltages V₃ and V₄ become: V₃=V_(C)+R_(C)I_(b1)+R_(C)I_(b1)Δ_(R), and V₄=Vc+R_(C)I_(b2)−R_(C)I_(b2)Δ_(R).

Therefore, a voltage applied to the passive element Z₁ becomes V₃−V₄=R_(C)(I_(b1)−I_(b2))+R_(C)(I_(b1)+I_(b2))Δ_(R). In other words, the signal components R_(C)(I_(b1)+I_(b2))Δ_(R) and R_(C)(I_(b1)−I_(b2)) are applied to the passive element Z₁. R_(C)(I_(b1)−I_(b2)) is a direct current offset voltage generated by deviation in bias current I_(b1) and I_(b2) contained in the current I₁ and I₂.

In the current amplifier circuit according to the present embodiment, since the passive element Z₁ is configured by the resistor RZ₁₁, the capacitor CZ₁, and the resistor RZ₁₂ connected in series, the direct current offset voltage is not converted into the current. Therefore, according to the present embodiment, the power consumption of the current amplifier circuit can be reduced.

In addition, the direct current offset voltage applied to the capacitor CZ₁ can be converted into the current by the voltage-current conversion circuit Gm, and supplied to the transistors M₁ and M₂. At this time, the current is desirably supplied such that a difference between the drain current I_(M1) of the transistor M₁ and the drain current I_(M2) of the transistor M₂ becomes small.

For example, when the voltage-current conversion coefficient is Gm=½R_(C), the output current of the voltage-current conversion circuit Gm becomes ±(I_(b1)−I_(b2))/2. When −(I_(b1)−I_(b2))/2 is supplied to the transistor M₁, the bias current of the transistor M₁ becomes I_(b1)−(I_(b1)−I_(b2))/2=(I_(b1)+I_(b2))/2. In addition, when +(I_(b1)−I_(b2))/2 is supplied to the transistor M₂, the bias current of the transistor M₂ becomes I_(b2)+(I_(b1)−I_(b2))/2=(I_(b1)+I_(b2))/2. In other words, the transistors M₁ and M₂ can be equalized in the bias current. Due to this, a direct current offset component contained in a difference between the current I_(M1) and the current I_(M2) respectively output from the output terminals T₃ and T₄ can be reduced.

Here, FIG. 48 is a diagram illustrating an example of the voltage-current conversion circuit Gm according to the present embodiment. As illustrated in FIG. 48, the voltage-current conversion circuit Gm includes a differential input circuit D, current mirror circuits CM₁ and CM₂, and current sources I_(GB2) and I_(GB3).

The differential input circuit D includes transistors M_(G1) and M_(G2), resistors R_(G1) and R_(G2), and a current source I_(GB1). The transistor M_(G1) is an NMOS transistor, having a drain terminal connected to the current mirror circuit CM₁, a gate terminal connected to the input terminal InP, and a source terminal connected to one end of the resistor R_(G1). The transistor M_(G2) is an NMOS transistor, having a drain terminal connected to the current mirror circuit CM₂, a gate terminal connected to the input terminal InM, and a source terminal connected to one end of the resistor R_(G2). The resistors R_(G1) and R_(G2) both have other ends connected to a high voltage side of the current source I_(GB1).

The current mirror circuit CM₁ is configured by transistors M_(G3) and M_(G6), and copies the drain current I_(MG1) of the transistor M_(G1). The transistors M_(G3) and M_(G6) are PMOS transistors, having the gate terminals connected to each other. The drain terminal of the transistor M_(G6) is connected to the output terminal OutP and a high voltage side of the current source I_(GB2). Due to this, a difference between the drain current I_(MG1) and the current I_(GB2) supplied by the current source I_(GB2), that is, the current corresponding to a change in voltage input from the input terminal InP is output from the output terminal OutP.

The current mirror circuit CM₂ is configured by transistors M_(G4) and M_(G5), and copies the drain current I_(MG2) of the transistor M_(G2). The transistors M_(G4) and M_(G5) are PMOS transistors, having the gate terminals connected to each other. The drain terminal of the transistor M_(G5) is connected to the output terminal OutM and a high voltage side of the current source I_(GB3). Due to this, a difference between the drain current I_(MG2) and the current I_(GB3) supplied by the current source I_(GB3), that is, the current corresponding to a change in voltage input from the input terminal InM is output from the output terminal OutM.

Here, by setting R_(G1)=R_(G2)=R_(C), I_(GB2)=I_(GB3)=I_(GB1)/2, and by setting transconductances gm_(MG1) and gm_(MG2) of the transistors M_(G1) and M_(G2) to 1/gm_(MG1)<<R_(C) and 1/gm_(MG2)<<R_(C), the voltage-current conversion coefficient Gm in the differential input circuit can be set to ½R_(C).

(Fourteenth Embodiment)

Next, a current amplifier circuit according to a fourteenth embodiment will be described with reference to FIGS. 49 and 50. Here, FIG. 49 is a diagram illustrating the current amplifier circuit according to the fourteenth embodiment. As illustrated in FIG. 49, the current amplifier circuit according to the present embodiment includes a transistor M₁, a resistor R₁, a transistor M₂, a resistor R₂, a control circuit, a passive element Z₁, and the voltage-current conversion circuit Gm. The voltage-current conversion circuit Gm of the present embodiment is different from the thirteenth embodiment in the connection method.

In the present embodiment, the output terminal OutP (the first output terminal) is connected to the other end of the resistor R₁ and the control circuit at a node N₁, and the output terminal OutM (the second output terminal) is connected to the other end of the resistor R₂ and the control circuit at a node N₂. The configurations of other components are same as the thirteenth embodiment.

In the present embodiment, a voltage V₃ at a node N₃ becomes V₃=V₁+R₁(I₁−I_(Gm)), and a voltage V₄ at a node N₄ becomes V₄=V₂+R₂(I₂+I_(Gm)). Here, −I_(Gm) and +I_(Gm) are the output current from the output terminals OutP and OutM of the voltage-current conversion circuit Gm. For example, in the case where R₁=R₂=R_(C), I₁=I_(b1)(1+Δ_(R)), I₂=I_(b2)(1−Δ_(R)), V₁=V₂=V_(C), the voltages V₃ and V₄ become: V₃=V_(C)+R_(C)I_(b1)−R_(C)I_(Gm)+R_(C)I_(b1)Δ_(R), and V₄=Vc+R_(C)I_(Ib2)+R_(C)I_(Gm)−R_(C)I_(b2)Δ_(R).

Therefore, a voltage applied to the passive element Z₁ becomes V₃−V₄=R_(C) (I_(b1)−I_(b2))−2R_(C)I_(Gm)+R_(C)(I_(b1)+I_(b2))Δ_(R). In other words, the signal component R_(C)(I_(b1)+I_(b2))Δ_(R), the direct current offset voltage R_(C)(I_(b1)−I_(b2)), and a voltage −2R_(C)I_(Gm) according to the output current of the voltage-current conversion circuit Gm are applied to the passive element Z₁.

Since I_(Gm) is obtained by converting the direct current offset voltage into the current by the voltage-current conversion coefficient Gm, a relation of I_(Gm)=Gm(R_(C)(I_(b1)−I_(b2))−2R_(C)I_(Gm)) is satisfied. In a case where Gm is sufficiently large and 2GmR_(C)>>1, the current I_(Gm) becomes: I_(Gm)≈(I_(b1)−I_(b2))/2, and the both bias currents flowing to the resistors R₁ and R₂ become equal to (I_(b1)+I_(b2))/2. Due to this, it is possible to reduce the direct current offset component contained in a difference between the current I_(M1) and the current I_(M2) output from output terminals T₃ and T₄ respectively.

Here, FIG. 50 is a diagram illustrating an example of the voltage-current conversion circuit Gm according to the present embodiment. As illustrated in FIG. 50, the voltage-current conversion circuit Gm includes the differential input circuit D, the current mirror circuits CM₁ and CM₂, and current sources I_(GB2) and I_(GB3). The voltage-current conversion circuit Gm of FIG. 50 is different from the voltage-current conversion circuit Gm of FIG. 48 in the configuration of the differential input circuit D.

The differential input circuit D includes transistors M_(G1) and M_(G2) and a current source I_(GB1). The transistor M_(G1) is an NMOS transistor, having a drain terminal connected to the current mirror circuit CM₁, a gate terminal connected to an input terminal InP, and a source terminal connected to a high voltage side of the current source I_(GB1). The transistor M_(G2) is an NMOS transistor, having a drain terminal connected to the current mirror circuit CM₂, a gate terminal connected to an input terminal InM, and a source terminal connected to a high voltage side of the current source I_(GB2). In other words, the voltage-current conversion circuit Gm of FIG. 50 is different from the voltage-current conversion circuit Gm of FIG. 48, and does not include the resistors R_(G1) and R_(G2).

Therefore, in the voltage-current conversion circuit Gm of FIG. 50, a change of the output current with respect to the change of the input voltage is large compared to the voltage-current conversion circuit Gm of FIG. 48. In other words, the voltage-current conversion coefficient Gm is large. This is because, unlike the case of the thirteenth embodiment, the voltage-current conversion coefficient Gm is desirably large in the case of the present embodiment.

(Fifteenth Embodiment)

Next, a current amplifier circuit according to a fifteenth embodiment will be described with reference to FIGS. 51 to 53. Here, FIG. 51 is a diagram illustrating the current amplifier circuit according to the fifteenth embodiment. As illustrated in FIG. 51, the current amplifier circuit according to the present embodiment includes a transistor M₁, a resistor R₁, a transistor M₂, a resistor R₂, a control circuit, the passive element Z₁, and a current-voltage conversion circuit. The configurations of the above components are same as the tenth embodiment. In the present embodiment, the current amplifier circuit further includes a comparison circuit and a current mirror circuit CM₃.

The comparison circuit compares the output voltages from output terminals T₃ and T₄ with a predetermined voltage (fourth predetermined voltage), and outputs the current according to the comparison result. The comparison circuit includes transistors M_(B1), M_(B2), M_(B3), and M_(B4), a resistor R_(B1), and current sources I_(B1) and I_(B2).

The transistor M_(B1) is a PMOS transistor, having a source terminal connected to one end of the resistor R_(B1) and a low voltage side of the current source I_(B1), a gate terminal from which a predetermined bias voltage V_(C3) is applied, and a drain terminal connected to the drain terminal of the transistor M_(B4). A bias voltage V_(C3) is set to be lower than the bias voltage of the output voltages from the output terminals T₃ and T₄.

The transistor M_(B2) is a PMOS transistor, having a source terminal connected to the other end of the resistor R_(B1), the source terminal of the transistor M_(B3), and a low voltage side of the current source I_(B2), a gate terminal connected to the output terminal T₄, and a drain terminal connected to the drain terminal of the transistor M_(B3) and the current mirror circuit CM₃.

The transistor M_(B3) is a PMOS transistor, having a source terminal connected to the other end of the resistor R_(B1), the source terminal of the transistor M_(B2), and the low voltage side of the current source I_(B2), a gate terminal connected to the output terminal T₃, a drain terminal connected to the drain terminal of the transistor M_(B2) and the current mirror circuit CM₃.

The transistor M_(B4) is an NMOS transistor, having a source terminal grounded, a drain terminal connected to the drain terminal of the transistor M_(B1), and a gate terminal connected to the drain terminal.

The current mirror circuit CM₃ includes transistors M_(B5), M_(B6), and M_(B7). The transistors M_(B5), M_(B6), and M_(B7) are NMOS transistors, each of which the gate terminal is connected to each other and the source terminal is grounded. In addition, the drain terminal of the transistor M_(B5) is connected to the drain terminals of the transistors M_(B2) and M_(B3), and the drain terminals of the transistors M_(B6) and M_(B7) are respectively connected to nodes N₄ and N₃.

Next, the operation of the current amplifier circuit according to the present embodiment will be described. In the following, the signal current is differentially received from input terminals T₁ and T₂.

First, the current-voltage conversion circuit converts the drain current I_(M1) and the drain current I_(M2) of the transistors M₁ and M₂ into voltages. The voltages converted by the current-voltage conversion circuit are output from the output terminals T₃ and T₄, and input to the comparison circuit.

Next, the comparison circuit compares the output voltages (the gate voltages of the transistors M_(B3) and M_(B2)) of the output terminals T₃ and T₄ with a predetermined voltage. In a case where the signal components contained in the output voltages are sufficiently small, that is, the output voltages are sufficiently higher than the predetermined voltage, all the current I_(B2) of the current source I_(B2) flows to the resistor R_(B1), the transistors M_(B3) and M_(B2) are turned off, and the current does not flow to the current mirror circuit CM₃.

On the other hand, in a case where the signal components contained in the output voltages are small and the output voltages are higher than the predetermined voltage but a difference between the voltage at the other end of the resistor R_(B1) and the voltage at one end of the resistor R_(B1) is smaller than R_(B1)I_(B2), a part of current of the current I_(B2) of the current source I_(B2) starts to flow to the transistors M_(B2) and M_(B3), and flows to the current mirror circuit CM₃. When the output voltages are further lowered and become smaller than the predetermined voltage, all the current I_(B2) of the current source I_(B2) and a part of the current I_(B1) of the current source I_(B1) start to flow to the transistors M_(B2) and M_(B3), and flow to the current mirror circuit CM₃. Thus, the current flowing to the current mirror circuit CM₃ is increased as the output voltages become small, that is, the signal components contained in the output voltages become large.

Since the output voltages are differentially input to the transistors M_(B2) and M_(B3), in a case where the signal components contained in the output voltages are sufficiently larger, one of the transistor M_(B2) or the transistor M_(B3) is turned on and the other one is turned off. Then, the drain current of the turned-on transistor is supplied to the transistor M_(B5).

The current output by the comparison circuit becomes the drain current I_(MB5) of the transistor M_(B5). The drain current I_(MB5) is copied by the transistors M_(B7) and M_(B6), and supplied from nodes N₃ and N₄ to the transistors M₁ and M₂.

With this configuration, in a case where the signal current contained in the current I₁ and I₂ is larger, the current amplifier circuit according to the present embodiment supplies the current to the transistors M₁ and M₂, and drive performance can be improved. Therefore, an amplification factor of the signal component can be improved.

As described in the first embodiment, the drain current I_(M1) of the transistor M₁ is I_(M1)=I₁+I_(Z1)=I_(b)+Δ_(R)I_(b)+2R_(C)I_(b)Δ_(R)/Z₁, the drain current I_(M2) flowing to the transistor M₂ is I_(M2)=I₂−I_(Z1)=I_(b)−Δ_(R)I_(b)−2R_(C)I_(b)Δ_(R)/Z₁. Therefore, in the current amplifier circuit of the first embodiment, the signal current contained in the output current from the output terminals T₃ and T₄ is not possible to become larger than the bias current I_(b).

However, according to the present embodiment, since the current can be supplied to the transistors M₁ and M₂ through the current mirror circuit CM₃, it is possible to make the signal current contained in the output current larger than the bias current I_(b).

In addition, the output current from the comparison circuit is smoothly changed by connecting the resistor R_(B1) between the transistor M_(B1) and the transistors M_(B2) and M_(B3), and the deformation of the output current from the comparison circuit can be suppressed.

FIG. 52 is a diagram illustrating another example of the current amplifier circuit according to the present embodiment. In FIG. 52, the drain terminal of the transistor M_(B2) of the comparison circuit and the drain terminal of the transistor M_(B3) are not connected, and they are connected to the current mirror circuit CM₃, respectively. The current mirror circuit CM₃ includes a first current mirror circuit CM₃₁ and a second current mirror circuit CM₃₂.

The first current mirror circuit CM₃₁ includes the transistor M_(B5) and the transistor M_(B6). The transistor M_(B5) has a drain terminal connected to a gate terminal and the drain terminal of the transistor M_(B2), the gate terminal connected to the gate terminal of the transistor M_(B6), and a source terminal grounded. The transistor M_(B6) has a drain terminal connected to a node N₄, a gate terminal connected to the gate terminal of the transistor M_(B5), and a source terminal grounded. A first current mirror circuit CM₃₁ copies the drain current I_(MB2) of the transistor M_(B2) and supplies the current from the node N₄ to the transistor M₂.

The second current mirror circuit CM₃₂ includes a transistor M_(B7) and a transistor M_(B8). The transistor M_(B8) has a drain terminal connected to a gate terminal and the drain terminal of the transistor M_(B3), the gate terminal connected to the gate terminal of the transistor M_(B7), and a source terminal grounded. The transistor M_(B7) has a drain terminal connected to a node N₃, a gate terminal connected to the gate terminal of the transistor M_(B8), and a source terminal grounded. A second current mirror circuit CM₃₂ copies the drain current I_(MB3) of the transistor M_(B3) and supplies the current from the node N₃ to the transistor M₁.

The configurations of other components are same as the current amplifier circuit of FIG. 51. With this configuration, it is possible to supply the current only to the transistor M₁ or the transistor M₂ outputting the lower voltage in the output voltages from the output terminals T₃ and T₄.

FIG. 53 is a diagram illustrating another example of the current amplifier circuit according to the present embodiment. In FIG. 53, the comparison circuit further includes a resistor R_(B2) and a current source I_(B3). The transistor M_(B2) has a source terminal connected to one end of the resistor R_(B1) and a lower voltage side of the current source I_(B2), a gate terminal connected to the output terminal T₄, and a drain terminal connected to the current mirror circuit CM₃₁. The transistor M_(B3) has a source terminal connected to one end of the resistor R_(B2) and a lower voltage side of the current source I_(B3), a gate terminal connected to the output terminal T₃, and a drain terminal connected to the current mirror circuit CM₃₂. The configurations of other components are same as FIG. 52.

With this configuration, it is possible to supply the current only to the transistor M₁ or the transistor M₂ outputting the lower voltage in the output voltages from the output terminals T₃ and T₄. In addition, it is possible to separately set a voltage to be used for comparison with the output voltage from the output terminal T₃ and a voltage to be used for comparison with the output voltage from the output terminal T₄. Furthermore, it is possible to separately set current to be supplied from the node N₃ and current to be supplied from the node N₄.

While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions. 

The invention claimed is:
 1. A current amplifier circuit, comprising: a first transistor having a first terminal, a second terminal, and a control terminal; a first resistor having one end connected to the first terminal of the first transistor; a second transistor having a first terminal, a second terminal, and a control terminal; a second resistor having one end connected to the first terminal of the second transistor; a first passive element connected between the first terminal of the first transistor and the first terminal of the second transistor; and a control circuit to control at least one of voltage at the control terminal of the first transistor and voltage at the control terminal of the second transistor such that voltage at an other end of the first resistor becomes equal to a first predetermined voltage and voltage at an other end of the second resistor becomes equal to a second predetermined voltage.
 2. The circuit according to claim 1, wherein the first predetermined voltage is equal to the second predetermined voltage.
 3. The circuit according to claim 1, further comprising: a third transistor having a first terminal, a second terminal, and a control terminal connected to the control terminal of the first transistor; a fourth transistor having a first terminal, a second terminal, and a control terminal connected to the control terminal of the second transistor; and a second passive element connected between the first terminal of the third transistor and the first terminal of the fourth transistor, wherein the second terminal of the third transistor is connected to the second terminal of the first transistor, and the second terminal of the fourth transistor is connected to the second terminal of the second transistor.
 4. The circuit according to claim 1, further comprising: a third transistor having a first terminal, a second terminal, and a control terminal connected to the first terminal of the first transistor; a fourth transistor having a first terminal, a second terminal, and a control terminal connected to the first terminal of the second transistor; and a second passive element connected between the first terminal of the third transistor and the first terminal of the fourth transistor, wherein the second terminal of the third transistor is connected to the second terminal of the first transistor, and the second terminal of the fourth transistor is connected to the second terminal of the second transistor.
 5. The circuit according to claim 1, comprising a switching circuit to switch a path such that current flowing in the first transistor and current flowing in the second transistor alternately is switched.
 6. The circuit according to claim 5, the switching circuit comprising: a first switch to switch the path between a path of current output from the second terminal of the first transistor and a path of current output from the second terminal of the second transistor; and a second switch to switch the path between a path of current received from the other end of the first resistor and a path of current received from the other end of the second resistor.
 7. The circuit according to claim 5, the switching circuit comprising: a first switch to switch the path between a path of current output from the second terminal of the first transistor and a path of current output from the second terminal of the second transistor; a third switch to switch the path between a path of current output from the one end of the first resistor and a path of current output from the one end of the second resistor; a fourth switch to switch a path of voltage received in the control circuit in order to control voltage at a gate terminal of the first transistor; and a fifth switch to switch a path of voltage received in the control circuit in order to control voltage at a gate terminal of the second transistor.
 8. The circuit according to claim 1, further comprising: a fifth transistor having a first terminal connected to a first input terminal, a second terminal connected to the other end of the first resistor, and a control terminal applied with a third predetermined voltage, and a sixth transistor having a first terminal connected to a second input terminal, a second terminal connected to the other end of the second resistor, and a control terminal applied with the third predetermined voltage.
 9. The circuit according to claim 1, wherein the first passive element includes a third resistor having one end connected to the first terminal of the first transistor, a fourth resistor having one end connected to the first terminal of the second transistor, and a capacitor connected between an other end of the third resistor and an other end of the fourth resistor, the circuit further comprising: a voltage-current conversion circuit having a first output terminal connected to the one end of the third resistor, a first input terminal connected to the other end of the third resistor, a second output terminal connected to the one end of the fourth resistor, and a second input terminal connected to the other end of the fourth resistor.
 10. The circuit according to claim 1, wherein the first passive element includes a third resistor having one end connected to the first terminal of the first transistor, a fourth resistor having one end connected to the first terminal of the second transistor, and a capacitor connected between an other end of the third resistor and an other end of the fourth resistor, the circuit further comprising: a voltage-current conversion circuit having a first output terminal connected to the other end of the first resistor, a first input terminal connected to the other end of the third resistor, a second output terminal connected to the other end of the second resistor, and a second input terminal connected to the other end of the fourth resistor.
 11. The circuit according to claim 1, further comprising: a current-voltage conversion circuit to convert drain current of the first transistor and the second transistor into voltage; a comparison circuit to compare the voltage converted by the current-voltage conversion circuit with a fourth predetermined voltage; and a current mirror circuit to supply current according to a comparison result of the comparison circuit to at least one of the first transistor and the second transistor.
 12. The circuit according to claim 1, wherein impedance of the first passive element is variable.
 13. The circuit according to claim 1, wherein the control terminal is a gate terminal or a base terminal, the first terminal is a source terminal or an emitter terminal, and the second terminal is a drain terminal or a collector terminal.
 14. An integrator, comprising: a current amplifier circuit according to claim 1; and a capacitor element to integrate an output signal of the current amplifier circuit.
 15. The integrator according to claim 14, wherein the capacitor element is formed including capacitor connected between the second terminal of the first transistor and the second terminal of the second transistor.
 16. The integrator according to claim 14, wherein the capacitor element is formed including a first capacitor connected to the second terminal of the first transistor, and a second capacitor connected to the second terminal of the second transistor.
 17. An AD converter, comprising: a current amplifier circuit according to claim 1; a current-voltage conversion circuit to convert an output signal of the current amplifier circuit to a voltage signal; and an AD conversion circuit to convert the voltage signal to a digital signal.
 18. A current amplifier circuit, comprising: a first transistor having a first terminal, a second terminal, and a control terminal; a first resistor having one end connected to the first terminal of the first transistor; a first passive element connected between the first terminal of the first transistor and reference potential; a control circuit to control voltage at the control terminal of the first transistor such that the other end of the first resistor becomes a first predetermined voltage: a second transistor having a first terminal connected to an input terminal, and a second terminal connected to an other end of the first resistor, and a control terminal applied with a second predetermined voltage.
 19. A current amplifier circuit, comprising: a first transistor having a first terminal, a second terminal, and a control terminal; a first resistor having one end connected to the first terminal of the first transistor; a first passive element connected between the first terminal of the first transistor and reference potential; and a control circuit to control voltage at the control terminal of the first transistor such that the other end of the first resistor becomes a first predetermined voltage, wherein the first passive element is formed including at least a capacitor. 